Glossary of Terms

I have tried where possible not to use technical terms and jargon, but this was not always possible. Where technical terms have been used (or everyday terms used in a more technical sense), I have tried to explain their meanings in situ, but here in addition is a glossary of such terms. The links within these definitions – where one definition relies to some extent on another, or where concepts are linked in some way – are to other terms on this page.

  • Absolute Time – The concept in philosophy and physics (sometimes referred to as Newtonian Time after its early proponent, Sir Isaac Newton) that time exists independently of any perceiver, and that it progresses at a consistent linear pace throughout the universe. Newton believed that absolute time was imperceptible and could only be understood mathematically.
  • Age – A period or division of time used in periodization. The length of an age varies depending on the time scale being measured (e.g. an age in the geological time scale is the smallest timespan, typically covering a few millions of years). Less technically, age is also the length of time that a person or object has lived or existed, or a particular stage in a life or existence.
  • Alarm Clock – A clock designed to wake a person at a specific time. Some water clocks and mechanical clocks were capable of chiming at a fixed time every day, but the first user-settable alarm clocks date back to 15th Century Europe, and the traditional mechanical wind-up alarm clock, settable for any time and small enough to use on a bedside table, appeared in Europe and the USA in the mid to late 19th Century.
  • Arrow of Time – The inherently directional nature of time, from a fixed immutable past towards an uncertain future. This apparent one-way direction, or time asymmetry, is what gives us an impression of time passing, and of our progressing through different moments. The main underlying reason for it, at least at the macroscopic level, is thought to be the Second Law of Thermodynamics, the statistical principle that things tend to progress in a direction of increasing entropy or disorder.
  • Atomic Clock – An extremely accurate timekeeping device or clock, regulated by the natural vibration frequency of atoms or molecules of substances like caesium, ammonia or hydrogen. Atomic clocks are the most accurate clocks we have, and are now used as primary standards for international time distribution services.
  • Barycentric Dynamical Time (TDB) – A time standard, intended mainly for astronomical use, which uses ephemeris (tables of the positions of celestial objects) and dynamical theory based on the barycentre (weighted average position) of the Solar System, but which also takes account of relativistic time dilation when calculating their orbits. It was introduced in the 1970s as a more accurate replacement for Ephemeris Time (ET), and has now itself been largely replaced by Terrestrial Time (TT).
  • Big Bang – The prevailing cosmological model for the early development of the universe, in which all the matter and energy of the entire universe was once squeezed into an infinitesimally small volume in an unimaginably hot and dense state. When this singularity erupted in a cataclysmic “explosion” some 13.8 billion years ago and began expanding rapidly everywhere at once, all of space, energy matter and time as we know it came into being.
  • Biological Clock – An endogenous (internal) timing mechanism in the body of all living organisms, responsible for regulating various cyclical responses, including eating, sleeping, hormone production, cellular regeneration, etc (in animals), and leaf movements, photosynthetic reactions, etc (in plants). The most important of these is the daily circadian clock, but there are also other peripheral biological clocks throughout the body.
  • Block Universe Theory of Time – The philosophical theory of time (also sometimes referred to as the block theory of time or the block time theory), which holds that the universe consists of an unchanging four-dimensional “block” of space-time, rather than three-dimensional space modulated by the passage of time, so that all points in timepast, present and future – are equally “real”. It is effectively equivalent to the doctrine of eternalism.
  • Calendar – A system of organizing days for social, religious, commercial or administrative purposes, typically by dividing time up into units of days, weeks, months and years. There are various different kinds of calendar, the most common being lunar calendars, solar calendars and luni-solar calendars.
  • Calendar Date – A particular day represented within a calendar system, the usual meaning of the word date.
  • Calendar Year – An approximation of a solar year in a given calendar system, the usual meaning of the word year.
  • Chronemics – The study of the use of time and the way that time is perceived and valued by individuals and cultures, particularly as regards non-verbal communication. The two main cultural approaches are monochronicity (where great value is placed on clock time and schedules, and tasks are typically done one at a time in a linear fashion) and polychronicity (a more fluid and less formal approach to scheduling time, where traditions and relationships are more important than tasks and schedules).
  • Chronobiology – The study in biology of the periodic or cyclic rhythms  of living organisms (particularly the daily circadian rhythms
  • Chronology – The science of arranging events in their order of occurrence in time, and ascertaining the dates and historical order of past events. It can be considered a part of periodization and of the study of history.
  • Chronomentrophobia – An irrational fear of clocks and watches. Related to chronophobia.
  • Chronometer – An exceptionally precise timepiece, designed to be accurate in all conditions of temperature, pressure, etc, especially one used at sea.
  • Chronometry – The art or science of the accurate measurement of time, also referred to as timekeeping. Chronometry typically includes both mechanical and electronic devices for the measurement of time, while horology usually refers to timekeeping using mechanical timepieces only.
  • Chronophobia – A psychological phobia which manifests itself as a persistent, abnormal and unwarranted fear of time or the passing of time. Most commonly experienced by prison inmates and the elderly.
  • Chronostasis – A kind of temporal illusion in which the first impression following the introduction of a new event or task demand to the brain appears to be extended in time, such as when the second hand of an analog clock appears to freeze in place for a short period of time after a person initially looks at it (it is also sometimes called the stopped clock illusion).
  • Chronotype – The natural variation in the timing of an individual’s circadian rhythm (by up to two hours or so either way). Some people (“larks” or morning people) tend to wake up early and go to bed early and are most alert during the first part of the day; others (“night owls” or evening people) are most alert in the late evening and prefer to go to bed late and wake up late.
  • Circadian Clock – A biochemical mechanism found in all animals, plants, fungi and even bacteria, which is coordinated with the day-night cycle, and which oscillates with a period of approximately 24 hours. It is the most important type of biological clock, and is used to regulate the body’s circadian rhythms. In animals, it is located in the suprachiasmatic nuclei region of the brain, and is regulated by Zeitgebers like the daily light/dark cycle.
  • Circadian Rhythm – The daily internal regulation of the timing of many biological processes, including sleeping, feeding, core body temperature, brain wave activity, hormone production, etc. It follows a rough 24-hour cycle and is regulated by the brain’s internal circadian clock.
  • Civil Time – The statutory time scale designated by civilian authorities to indicate the official local time. This is generally Standard Time in a particular time zone, which is set as a fixed offset from Coordinated Universal Time (UTC) or Greenwich Mean Time (GMT), possibly adjusted by Daylight Saving Time (DST) during part of the year.
  • Clepsydra – The Greek word for water clock.
  • Clock – Any free-standing device or instrument for measuring and displaying the time. A chronometer and a watch are specific types of clock, as are sundials, water clocks, mechanical clocks, atomic clocks, etc. A timepiece usually refers to a silent clock that does not strike or otherwise audibly mark the hours.
  • Clock Time – A reading of a point in time as given by a clock, the usual measure of the passing of time that most people use in daily life. It is a term usually used in contradistinction to psychological time or computer system time.
  • Coordinated Universal Time (UTC) – The primary time standard by which the world’s civilian authorities regulate their clocks and time measurement. With adjustments for different time zones, it provides the basis for Standard Time or civil time, a role it took over from the slightly less accurate Greenwich Mean Time (GMT) in 1964. It is the main variant of Universal Time (UT), and differs from International Atomic Time (TAI) only by an integer number of leap seconds added at intervals to compensate for the slowing of Earth’s rotation and natural cataclysms like earthquakes and hurricanes.
  • Coordinate Time – The time specified by the time coordinate of a space-time coordinate system, relative to some implied observer. It measures the apparent time change between two moving objects as perceived objectively and independently by a distant observer, as opposed to the subjective or proper time experienced by the moving objects themselves (in relativistic time, particularly at high speeds, these will be different).
  • Cosmological Time Scale – The time scale used in periodization that is appropriate to cosmological events billions of years in the past or future, and therefore outside the human time scale and even the geological time scale of the Earth. It typically uses the Big Bang (the creation of the universe itself) as a reference point.
  • Cyclic View of Time – Many ancient cultures and religions (including Hindus, Buddhists, Jains, the Babylonians, Ancient Greeks, Incas, Mayas, Hopi and other Native American Tribes) believed that time was cyclic, not linear, often envisioned as an ever-turning “wheel of time” or Kalachakra, in which there were repeating ages over the infinite life (hypertime) of the universe.
  • Date – This usually refers to a calendar date, a particular day represented within a calendar system. A particular date is usually specified according to unit divisions like days, months and years and a starting reference point or fiduciary epoch (e.g. 3rd of May 2013CE).
  • Day – A unit of measurement of time equal to 24 hours, corresponding to a complete rotation of the Earth on its axis. 7 days make up a week; 28, 29, 30 or 31 days make up a month (depending on the calendar); and 365 or 366 days make up a year (depending on whether or not it is a leap year). More informally, day is also used to refer to the part of the 24-hour period between sunrise and sunset when it is light (daytime).
  • Daylight Saving Time (DST) – A periodic adjustment to Standard Time adopted by some countries whereby clocks are advanced by one hour during the lighter summer months so that evenings have more apparent daylight and mornings have less. DST (also known as “summer time”) was first implemented during the First World War, and it is observed today by most of Europe and North America.
  • Deep Time – The long-term history or prehistory of the Earth, as measure by the geological time scale, covering periods of many millions or billions of years. Events and periods on this scale can be ascertained by the study of rock layers (stratigraphy) and major events like mass species extinctions.
  • Duration – Informally, duration is the measure of continuance of any object or event within time. In philosophy, it refers more specifically to Henri Bergson’s theory of subjective and ineffable time that can only be grasped through a simple intuition of the imagination.
  • Dynamical Time – A time scale that is defined implicitly, i.e. inferred from the observed position of an astronomical object according to a theory of its motion. Ephemeris Time (ET) was the first such system in regular use, but this has since been replaced by Barycentric Dynamical Time (BDT) and particularly by Terrestrial Time (TT).
  • Electric Clock – A clock powered by electricity rather than a hanging weight or a spring), although it usually refers to the electrically-powered mechanical clocks that were used before electronic clocks replaced them in the 1980s. They were first introduced in the 1840s, and became the most widely used type of clock by the 1930s.
  • Electronic Clock – A clock powered and governed by electronics, in practice usually meaning a quartz clock.
  • Embolism – Another word for Intercalation.
  • Endurantism – The philosophical theory that objects persist through time as complete three-dimensional individuals at every moment of their existence, in contradistinction to the less mainstream or conventional view of perdurantism. According to endurantism, each instance of an object’s existence is fundamentally separate from the other previous and future instances. This view is generally (although not necessarily) consistent with presentism.
  • Eon – A period or division of time used in periodization. The length of an eon varies depending on the time scale being measured (e.g. an eon in the geological time scale refers to very broad timespans covering half a billion years or more, and is divided up into the more commonly used division of eras).
  • Ephemeris Time (ET) – A time standard in which the time and time scale are inferred from the observed positions of astronomical objects in the sky, using the dynamical theory of their motions. Barycentric Dynamical Time (TDB) and Terrestrial Time (TT) are more accurate replacements for ephemeris time that take account of relativistic time dilation.
  • Epoch – A period or division of time used in periodization. The length of an epoch varies depending on the time scale being measured (e.g. an epoch in the geological time scale is an intermediate timespan, typically covering tens of millions of years, between a period and an age). A fiduciary epoch is also sometimes referred to simply as an epoch.
  • Era – A period or division of time used in periodization. The length of an era varies depending on the time scale being measured (e.g. an era in the geological time scale is an intermediate timespan, typically covering several hundred million years, between an eon and a period).
  • Escapement Mechanism – A device in clocks and watches, usually consisting of an escape wheel and anchor, that greatly increases the accuracy of their timekeeping. Typically, an escapement transmits a periodic impulse from the spring or weight to the balance wheel or pendulum, by allowing a tooth to escape from a pallet at regular intervals.
  • Eternalism – The philosophical view of time that holds that all points in time are equally “real”, and that past and future events exist in a very real way, even if they cannot be experienced at will. According to this view (and in contrast with the opposing viewpoint of presentism), the “flow of time” we perceive is just an illusion of consciousness, because in reality all of time is always there. This is consistent with the block universe theory of time.
  • Eternity – Infinite or unending time, a duration without beginning or end, or a timeless state to which time just does not apply. The idea of infinite time goes back to Aristotle in ancient Greece, but it also accords with some views of modern physics.
  • Event – An object, physical situation or occurrence in time. Or, from the point of view of relativistic physics, a particular location in space-time (i.e. a point in space at an instant in time). Space-time as a whole is a collection of an infinite number of events.
  • Fiduciary Epoch – The starting or reference point of a particular calendar system (also known simply as an epoch), such as the birth of Christ in the Gregorian calendar, or the founding of Rome in the old Roman calendar. Prior to the use of fiduciary epochs, dates were usually reckoned using regnal years.
  • FutureTime, or a period of time, which is still to come after the present moment. The future may be seen as fixed and predetermined (as in the philosophical doctrine of eternalism), or, more usually, as essentially unknown and perhaps unknowable, and open to many different possibilities and permutations.
  • Futurology – The study or prediction of the future of mankind, particularly of trends or developments in science, technology, political or social structure, etc. Futurology claims to study the future in much the same way that history studies the past, although it necessarily relies on uncertain prediction and conjecture to some extent.
  • Geological Time Scale – The time scale used in periodization that is appropriate to deep time, outside the human time scale, covering periods and events in the history of the Earth of the magnitude of many millions or even billions of years. In the absence of human or archaeological markers, the geological (or geologic) time scale utilizes stratigraphy and major events like mass species extinctions.
  • Global Positioning System (GPS) – A system of satellites, computers and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver. It provides extremely accurate location and time information to military, civil and commercial users around the world. Also see GPS Time.
  • Gnomon – The projecting part of a sundial that casts the shadow, and that indicates the hour of the day by the position of its shadow. Traditionally, a gnomon is often triangular in shape (or some variation thereof), but any kind of a pin or rod will serve. A gnomon should be positioned parallel to the Earth’s axis on the sundial so that the hour lines indicate equal hours on any day of the year.
  • GPS Time – A time standard used by the Global Positioning System (GPS), maintained by extremely accurate atomic clocks on a system of satellites orbiting the Earth. GPS time remains at a constant offset of 19 seconds with International Atomic Time (TAI) but, because it is not corrected to match the exact rotation of the Earth, it does not contain leap seconds or other corrections that are made to Coordinated Universal Time (UTC), so that periodic corrections need to be made to the on-board clocks to keep them synchronized with ground clocks.
  • Greenwich Mean Time (GMT) – The earliest internationally accepted time standard, established in 1884, although it was superseded by Universal Time (UT) in 1928. It is a telescope-based standard, and refers to the mean solar time at the Royal Observatory in Greenwich, London. For the most part, it is equivalent to Universal Time (UT) and Coordinated Universal Time (UTC). Under Standard Time, all time zones across the world are still defined in terms of offsets from GMT/UTC.
  • Growing Block Universe Theory of Time – A variation of the philosophical theory of eternalism, in which more and more of the world comes into being with the passage of time (hence, the “block universe” is said to be “growing”). According to this view, sometimes known simply as the growing block view, the past and present have a definite existence, but the future is not yet part of this universe and therefore cannot be said to exist.
  • History – The study of the past, and particularly of how it relates to humans. Chronology, periodization and the interpretation of the past are together known as the study of history.
  • Horology – The science or art of making timepieces and of measuring time, particularly using mechanical clocks.
  • Hour – A unit of measurement of time, comprising 60 minutes or 3,600 seconds. 24 hours make up a day. Although it is not itself a standard unit defined by the International System of Units (SI), the hour is a unit accepted for use with SI.
  • Human Time Scale – The time scale used in periodization that is used to divide up human history into convenient periods or blocks of time, usually with reference to calendar dates, prominent individuals of a period, historical or political events, or cultural movements.
  • Hypertime – A fictional or theoretical time-like construct that extends beyond normal time, encompassing or spanning many (possibly infinitely many) distinct timelines. For example, in a cyclic view of time the history of the world repeats itself over and over again throughout the infinite life of the universe, presupposing an overall linear ordering within some kind of hypertime.
  • Imaginary Time – A concept in theoretical physics that bears a similar relationship to normal physical time as the imaginary number scale does to the real numbers in the complex plane. It can perhaps best be portrayed as an axis running perpendicular to that of regular time. Although rather abstract and difficult to visualize, imaginary time is not imaginary in the sense of being unreal or made-up, and it can help mathematically to smooth out gravitational singularities in models of the universe.
  • Immortality – Eternal life, or the ability to live forever. Some scientists believe that human immortality is achievable in the next few decades, but all avenues being currently explored remain experimental or hypothetical.
  • Intercalation – The insertion of leap days or leap months (or both) into some calendar years in order to synchronize the calendar to the seasons of the year or the phases of the Moon. This arises because a solar year is not an exact number of days (about 365.242 days), a lunar month is not an exact number of days (about 29.53 days), and a solar year is not an exact number of lunar months (about 12.3685 lunar months). Intercalation is also called embolism.
  • Internal Clock – An informal name for a neurological timing mechanism in the brain (distinct from the biological clock or circadian clock, which is also sometimes referred to as the internal clock), typically used to time durations in the seconds-to-minutes range. Specific neurons near the base of the brain become active when a person is asked to estimate a duration of time, which in turn trigger other cells in the frontal cortex to allow us to judge the passing of time. The process appears to be specifically linked to levels of the neurotransmitters dopamine and norepinephrine in the brain.
  • International Atomic Time (TAI) – A high-precision time standard, used as the basis for Coordinated Universal Time (which is used for civil timekeeping throughout the world) and for Terrestrial Time (the main time standard used for astronomical calculations). TAI is a weighted average of the time kept by over 200 atomic clocks worldwide, corrected for height above sea level, resulting in far more stability and accuracy than a reading from any single clock.
  • International System of Units (SI) – The modern form of the metric system of measurement, and the world’s most widely-used system of measurement, both in everyday commerce and in science. Established in 1960, it is based on the metre-kilogram-second (MKS) system of measurement. The SI base unit for time is the second (abbreviated as s or sec), and the other SI units are: metre for length, kilogram for mass, ampere for electric current, kelvin for temperature, candela for luminous intensity, and mole for the amount of substance.
  • Interval – The duration of time between two events, or the period of time marked off by two events.
  • IsochronismEvents that occur regularly or at equal intervals of time, an essential property for timekeeping and the construction of clocks. For instance, in horology, the oscillation period of the pendulum in a pendulum clock is constant, regardless of the angle of the swing. A mechanical clock or watch needs to be isochronous if it is to run at the same rate regardless of changes in its drive force.
  • Kappa Effect – A temporal illusion in the perception of time when the temporal duration between a sequence of consecutive stimuli is perceived to be relatively longer or shorter than its actual elapsed time, due to the spatial separation between each consecutive stimuli. It is thought to occur because the brain appears to be hard-wired to expect temporal intervals that would produce constant velocity (i.e. uniform motion).
  • Mean Solar Time – An idealized averaged solar time (actually calculated from sidereal time) that would be measured by observation if the Sun travelled at a uniform apparent speed throughout the year. It may differ from apparent solar time by up to 16 minutes.
  • Mechanical Clock – A mechanical device that uses continually repeated mechanical (“clockwork”) motion to measure or indicate the passage of time. Mechanical clocks began to be developed independently in China, the Middle East and Europe during the early Middle Ages, using a simple controlled release of power or escapement mechanism, as well as all manner of toothed wheels, ratchets, gears and oscillating levers.
  • Mental Chronometry – A technique in experimental and cognitive psychology to assess how fast an individual can execute certain mental operations. A person’s reaction time (the elapsed time between the presentation of a sensory stimulus and their subsequent behavioural response, such as the pressing of a button) is used as a measure of cognitive processing speed and efficiency.
  • Metric Time – The measurement of time intervals using the metric system (multiples and powers of 10, with metric prefixes such as kiloseconds, milliseconds, etc). A 10-hour clock was briefly popular in revolutionary France at the end of the 18th Century, and computers use various types of metric time in their generation of System Time, but metric time has not gained much traction for general use.
  • Minute – A unit of measurement of time, comprising 60 seconds. 60 minutes make up one hour. Although it is not itself a standard unit defined by the International System of Units (SI), the minute is a unit accepted for use with SI.
  • Month – A unit of measurement of time, roughly based on the period of a complete revolution of the Moon around the Earth (lunar month). There may be anywhere from 28 to 31 days in a month, depending on the kind of calendar used, and twelve or thirteen months in a year.
  • Monochronicity – An attitude to time and time use where great value is placed on clock time and schedules, and tasks are typically done one at a time in a linear fashion. In the study of chronemics, monochronicity stands in contradistinction to polychronicity.
  • Oddball Effect – A temporal illusion that occurs when the brain experiences something unusual or out of the normal run of events. A person’s perception of time appears to slow down when they perceive an unusual or stressful sensory stimulus, as the brain takes more time to process the event and to record additional information. For example, time seems to slow down when a person skydives or bungee jumps, or when a person suddenly and unexpectedly senses the presence of a potential threat or mate.
  • Oscillatory Motion – The repeated and regular variation of some measure about some mean value, whether it be swinging from a point of equilibrium, fluctuation between two different atomic states, etc. This is the phenomenon utilized in some way by most clocks, e.g. the harmonic motion of a simple pendulum, molecular vibrations, or even biological rhythms.
  • Past – The totality of events occurring before a given point in time (the present), events which are usually considered to be fixed and immutable. In contrast with the future, the past can be accessed through memory or though written records. The study of the past is known as history.
  • Pendulum Clock – A type of mechanical clock that uses a pendulum (i.e. a swinging weight), as its timekeeping element. Invented by Christiaan Huygens in 1656, the pendulum clock became widely used, and was the world’s most precise timekeeper until the invention of the quartz clock in 1927.
  • Perdurantism – The philosophical theory, in contradistinction to endurantism, that holds that any object that continues to exist through time does so as a continuous reality, and the thing as a whole is then the sum of all of its temporal parts, or instances of existing. This view is generally (although not necessarily) consistent with eternalism.
  • Period – A timespan or division of time used in periodization. The length of a period varies depending on the time scale being measured (e.g. a period in the geological time scale is an intermediate timespan, typically covering tens or hundreds of millions of years, between an era and an epoch). A period is also the time taken for one full oscillation of a pendulum or other oscillating or cyclic phenomenon, as well as its more general meaning as a length or portion or block of time.
  • Periodization – The division of the past into convenient periods or blocks of time, using human records for the more recent past of the Human Time Scale, radiocarbon or even longer-scale radiometric dating techniques for the longer-term Geological Time Scale, and physical theories and extrapolations for the even longer-term Cosmological Time Scale. Grouping together periods of time with relatively stable characteristics in this way provides at least some sort of framework to help us understand what would otherwise be a continuous stream of scattered and apparently random events.
  • Peripheral Biological Clocks – Secondary biological clocks, located throughout the body (e.g. in the liver, heart, pancreas, kidneys, lungs, intestines, skin, lymphocytes, etc), all of which show natural daily oscillations, and which to some extent regulate the timing of the body’s natural processes. Unlike the main circadian clock, these organs are entrained independently by Zeitgebers like the timing of meals, ambient temperatures, etc, rather than by the light-dark cycle.
  • Physical TimeTime as defined by the science of physics, whether that be Newtonian absolute time or Einsteinian relativistic time. The phrase physical time, though, is perhaps most often used to mean objectively measured and experienced clock time in contradistinction to subjective psychological time.
  • Polychronicity – An attitude to time and time use characterized by a fluid and informal approach to scheduling and timekeeping, and in which traditions and relationships are more important than tasks and schedules. In the study of chronemics, polychronicity stands in contradistinction to monochronicity.
  • Prehistory – The span of time before recorded history and the invention of writing systems, also sometimes referred to as “deep time”. With no human or archaeological markers to use, other techniques like stratigraphy and events like mass extinctions must be utilized to map out prehistory on the geological time scale.
  • Present – The time associated with those events being perceived directly and for the first time, and not as a recollection of the past or a speculation of the future. It is equivalent to the word “now”, and is that time located between the past and the future. It is usually considered to be an infinitesimal or durationless moment, although, depending on how the word is being used, it can extend to a day or even a whole era of time (also see the specious present).
  • Presentism – The philosophical view of time (in contrast with the opposing viewpoint of eternalism) that only events and entities that occur in the present actually exist. Therefore, things come into existence and then drop out of existence, and past events or entities that cannot be experienced now literally do not exist. Also, because the future is indeterminate or merely potential, it cannot be said to exist either.
  • Proper Time – The elapsed time between two events as measured by a clock that passes through both events. According to the Theory of Relativity and relativistic time, this is different from the independently measured coordinate time, especially at relativistic speeds.
  • Psychological Time – The subjective and potentially malleable perception of the passage of time, which can differ significantly between individuals and/or in different circumstances. This malleability of time perception is made abundantly apparent by the various temporal illusions we experience. The concept of psychological time is often used in contradistinction to physical time or clock time.
  • Quartz Clock – A clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. Quartz crystals oscillate with very precise frequency, so that quartz clocks are much more accurate than mechanical clocks. First introduced in the 1920s, they benefited greatly from the advances in microelectronics in the 1960s, and had become the dominant timekeeping technology for both clocks and watches by the 1980s.
  • Regnal Year – A year of the reign of a sovereign or leader, used to identify specific years in history. This ancient system of reckoning dates was the norm before fiduciary epochs were established for calendars.
  • Relational Time – The concept in philosophy (first described by Gottfried Leibniz) that time does not refer to any actual existing dimension that events and objects move through in some way (as in absolute time). Rather, time is a purely intellectual concept that enables us to sequence and compare events, which only has any meaning if there are objects with which it can interact or relate. Time (and space), then, are merely products of the way we represent things to ourselves, that being only way we are truly capable of knowing objects and events.
  • Relative Time – The concept in philosophy and physics (first described by Sir Isaac Newton) that we, as humans, are only capable of perceiving time as a relative measurement of perceivable objects in motion (like the Moon or Sun) from which we infer the passage of time, even if the reality of time is the theoretical and mathematical model of absolute time.
  • Relativistic Time – The idea in physics, arising from Albert Einstein’s Theory of Relativity, that time is not the traditional absolute time of Newton, but just one dimension of (dynamically curved) space-time. In this scenario, time is an integral part of the very fabric of the universe (and cannot exist apart from the universe), but both space and time are necessarily flexible and relative, due to effects such as time dilation. According to this theory, time travel is a distinct theoretical possibility, although it has never been achieved in practice.
  • Second – A unit of measurement of time, and the base unit of time in the International System of Units (SI). 60 seconds make up one minute, and there are 3,600 seconds in one hour. Originally, a second was defined as 1/86,400 of a day as measured by mean solar time, although since 1967 it has been defined in terms of atomic vibrations, as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”.
  • Shadow Clock – Another name for Sundial.
  • Sidereal TimeTime measurement based on the rotation of the Earth on its axis, but using the motion of the “fixed stars” (rather than the Sun) across the sky as the Earth rotates as the basis for time determination. It is mainly used for astronomical purposes, but it is also used in the calculation of Mean Solar Time.
  • Simultaneity – The property of two events happening at the same time in any particular frame of reference. According to relativistic physics, however, simultaneity is NOT an absolute property between events, and so what is simultaneous in one frame of reference will not necessarily be simultaneous in another. For objects moving at normal everyday speeds, though, simultaneity is an adequate approximation.
  • SI System – A common abbreviation for International System of Units.
  • Solar Calendar – A calendar based on, and synchronized to, the apparent motion of the Sun over the year. It makes no attempt to match the changes in the Moon, and the division of the year into months is therefore purely nominal. However, a solar calendar (unlike a lunar calendar) does remain in line with annual seasonal changes.
  • Solar Year – The usual way of measuring a year, based on the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from the Earth (e.g. from vernal equinox to vernal equinox, or from summer solstice to summer solstice). A solar year, also known as a tropical year, is approximately 365.242 days.
  • Solar TimeTime measurement based on the rotation of the Earth on its axis, using the Sun’s apparent motion across the sky to measure the duration of a day. Because the Earth’s orbit is an ellipse rather than a circle, its speed around the Sun is not constant, so that a day in apparent solar time is not always exactly 24 hours long. Mean Solar Time is used as an idealized averaged solar time.
  • Space-time – The mathematical model (also written spacetime, unhyphenated) at the heart of the Theory of Relativity, that combines space and time into a single interwoven continuum (three dimensions of space and one of time). Space-time as a whole is a collection of an infinite number of events (points in space at instants in time). Under this model, both the past and the future are simply “there”, laid out as part of four-dimensional space-time, which is consistent with the philosophical view of eternalism or the block universe theory of time.
  • Specious Present – The philosophical notion that the present is not instantaneous and durationless, but merely the most recent part of the past. William James called the specious present “the short duration of which we are immediately and incessantly sensible”, and argued that anything we are conscious of as having an earlier part and a later part cannot possibly be durationless, but must take place over at least a short period of physical time.
  • Standard Time (ST) – The familiar clock time most people use in everyday life. It is technically defined in terms of offsets from Coordinated Universal Time (UTC) (originally from Greenwich Mean Time), and is based on the division of the Earth’s sphere into 24 equal time zones according to the distance east or west of the Prime Meridian in Greenwich, England, sometimes adjusted by Daylight Saving Time (DST). Standard Time was first formalized by the Scottish-Canadian Sir Sandford Fleming in 1879, and is now used throughout the world as the basis for local civil time.
  • Stopped Clock Illusion – Another name for Chronostasis.
  • Stratigraphy – The branch of geology which studies sedimentary and volcanic rock layers (strata) and layering (stratification), which can then be used in the periodization of the the geological time scale.
  • Sundial – A device, or early form of clock, which measures the time of day by means of a shadow cast by the Sun onto a cylindrical stone, to give a reasonably accurate reading of the local solar time. The projecting pin, rod or shape that actually casts the shadow is known as a gnomon. The sundial (also known as “shadow clock”) was widely used in ancient times in almost all cultures.
  • Suprachiasmatic Nucleus – A tiny pinhead-sized area in the hypothalamus region of the basal forebrain of animals which constitutes the body’s circadian clock. It is responsible for the overall circadian rhythms that the body perceives internally, which regulate essential functions like sleep, feeding patterns, alertness, core body temperature, brain wave activity, hormone production, etc, according to a daily schedule. There is one nucleus in each hemisphere of the brain, and so collectively they are referred to as the suprachiasmatic nuclei.
  • Synchronicity – The concept in psychology, first described by the Swiss psychologist Carl Jung in the 1920s, whereby a person experiences two or more events at different times as meaningfully related in some way, even though they are unlikely to be causally related. More informally, it is used to refer to simultaneity or coincidence in time.
  • Synodic Month – The time between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth, the usual meaning of Lunar Month.
  • System Time – The time standard used by computers, which typically works by counting regular “ticks” since some arbitrary starting date, called the epoch. Examples include UNIX time, FILETIME, OpenVMS, RISC OS, etc. Computer system clocks use a variety of metric time, and most use an Internet protocol  called Network Time Protocol (NTP) to coordinate system time with standard Coordinated Universal Time (UTC).
  • Temporal Finitism – The philosophical and religious doctrine that time is finite in nature, and had a distinct beginning at the time of Creation, and will come to an equally distinct end in the so-called “end-times”. The idea arose in the early Middle Ages, when Christian, Muslim and Jewish philosophers and theologians were unable to reconcile the popular Aristotelian conception of eternity with the Abrahamic view of the creation of the universe by God.
  • Temporal Illusion – A distortion in the perception of time that occurs for various reasons, often resulting from different kinds of stress. As a result, time may be momentarily perceived as slowing down, stopping, speeding up, or even running backwards, as the timing and temporal order of events are misperceived by the brain. Examples of temporal illusions are the kappa effect, the oddball effect, chronostasis, etc.
  • Temporal Parts – The different parts or instances of an object that exist at various points in time. According to the philosophical view of perdurantism on the persistence of material objects, an object as a whole is the sum of all of its temporal parts. Some of the temporal parts of a particular person, for example, include their childhood, middle age and old age.
  • Tensed Theory of Time (A-Theory) – The philosophical doctrine that events are normally characterized as past, present or future in natural language by the verbal inflection of tenses, and that these differences are fundamental and represent the irreducible foundations of temporality. By extension, A-theorists deny that the past, present and future are equally real and existent (as the tenseless theory of time contends), an argument generally consistent with presentism.
  • Tenseless Theory of Time (B-Theory) – The philosophical doctrine that events can be described as “earlier than”, “simultaneous with” or “later than” others, without the need for verb tenses as the tensed theory of time contends. By extension, B-theorists believe that the past, the present and the future are all equally real and existent, an argument consistent with eternalism and the block universe theory of time.
  • Terrestrial Time (TT) – A modern astronomical time standard, used primarily for time measurements of astronomical observations made from the surface of the Earth. It is an implicitly defined dynamical time standard and, since the 1970s, it has replaced earlier flawed dynamical time standards like Ephemeris Time (ET), Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT). TT differs from International Atomic Time (TAI) by about 32 seconds, although it is not actually based on atomic clocks.
  • Time – A dimension in which events can be ordered from the past through the present and into the future, and also the measure of durations of events and the intervals between them. Time can be seen as the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole.
  • Time Asymmetry (T-asymmetry) – The inherent one-way directionality of time, often referred to as the arrow of time, which is what gives us an impression of time passing or flowing. The main underlying reason for the time asymmetry we observe at the macroscopic level is the Second Law of Thermodynamics, one of the only time-asymmetric laws in physics. In reality, though, it is more a statistical principle than a fundamental law, and so time asymmetry occurs not because the reverse is impossible, but because it is highly unlikely.
  • Time Banking – A complementary monetary system, developed in the 1980s and since established in various countries throughout the world, that uses units of time as currency. The unit of currency is an hour’s worth of any person’s labour, generally known as a “time dollar” or a “time credit”. The idea is that time spent on work that does not normally provide monetary benefits (e.g. mentoring children, caring for the elderly, being neighbourly, etc) earns credits that can then be spent to receive other services of different types.
  • Time Dilation – The effect in relativistic time whereby rates of time actually run differently depending on relative motion. Therefore, time effectively passes at different speeds for different observers travelling at different speeds, and two synchronized clocks will not necessarily stay synchronized if they move relative to each other. This effect, although measurable, is negligible at everyday speeds, but it becomes it much more pronounced at relativistic speeds close to the speed of light, or in conditions of very high gravity. One corollary of time dilation is the theoretical possibility of time travel.
  • Time Discipline – The field in sociology and anthropology, pioneered by E.P. Thompson in the 1960s, which looks at the economic rules, conventions, customs, and expectations governing the measurement of time in different societies and throughout history.
  • Timekeeping – Another word for Chronometry.
  • Time Management – The process of planning and organizing tasks or events with a view exercising conscious control over the amount of time spent on specific activities, usually in an attempt to increase effectiveness, efficiency or productivity. First developed in the early 20th Century, it is an important aspect of business and project management, but increasingly it is also being applied to education and personal activities.
  • Time Orientation – A cultural or national preference toward past, present, or future thinking, with a concomitant effect on how that culture values time, and the extent to which it believes time can be controlled. America is often considered to be future-orientated, for example, while older cultures like Britain and India are more past-orientated.
  • Time Perception – The subjective experience of the passage of time or the perceived duration of events, which can differ significantly between individuals and/or in different circumstances. Although physical time appears to be objective, psychological time is very much subjective and potentially malleable, as exemplified by the various temporal illusions we experience.
  • Timepiece – An apparatus for measuring and recording the progress of time. Timepiece is usually synonymous with clock, although it typically refers to a silent clock that does not strike or otherwise audibly mark the hours, and may also encompass portable instruments like watches.
  • Time Scale – Any method for measuring divisions of time, or for specifying the rate at which time passes. This may refer to measurement systems such as the human time scale, the geological time scale and the cosmological time scale used in periodization, but it may also refer to one of the official time standards used in the measurement of time.
  • Timestamp – A sequence of characters or encoded information identifying when a certain event occurred, usually giving a date and a time of day. An example is a postmark on a letter, but these days it is more likely to refer to digital date and time information embedded in computer files or digital pictures.
  • Time Standard – Any officially-recognized specification of an accepted standard of time measurement. Examples include International Atomic Time (TAI), Coordinated Universal Time (UTC), Greenwich Mean Time (GMT) and Terrestrial Time (TT), but there are many others.
  • Time Symmetry (T-symmetry) – The theoretical symmetry of most physical laws under a time-reversal transformation. In general, physical processes and laws at the microscopic level – whether classical, relativistic or quantum – are time-symmetric, so that, if a process is physically possible, then so is the same process run backwards. Most of the laws of physics do not therefore necessarily specify an arrow of time, although the Second Law of Thermodynamics DOES appear to be time-asymmetric, and is thought to be the main cause of the observed arrow of time at the macroscopic level.
  • Time Travel – The possibility in relativistic time of movement backwards and/or forwards in time, independent of the flow of time we normally observe on Earth, in much the same way as we can move between different points in space. While theoretically allowed by the Theory of Relativity, time travel has never been achieved in practice, and many believe that unavoidable paradoxes will forever preclude practical time travel.
  • Time Zone – A region that has a uniform civil time for legal, commercial and social purposes, i.e. the usual clock time that most people use in daily life. Standard Time divides the world into 24 time zones, each one covering 15 degrees of longitude (although, for reasons of practicality, some zones also follow country boundaries for part of their length), with the time for that zone defined in terms of one hour offsets (or, in rare cases, half-hour or even quarter-hour offsets) from Coordinated Universal Time (UTC)/Greenwich Mean Time (GMT).
  • Tropical Year – Another word for Solar Year.
  • Watch – A type of portable timepiece or clock, most commonly worn on the wrist, although early fob watches were attached on a chain and carried in a pocket. Watches evolved in the 17th Century, and the first watches were strictly mechanical, although as technology has progressed, spring-driven mechanisms have largely been replaced by the more accurate quartz vibrations used in quartz clocks.
  • Water Clock – A kind of timepiece or clock in which time is measured by the regulated flow of liquid into or out of a vessel. Among the oldest of all time-measuring instruments, the water clock (also known as clepsydra in Greek) was also the most accurate type of clock until mechanical clocks were developed in the Middle Ages.
  • Week – A unit of time equal to 7 days. It is the standard time period used for cycles of work and rest days in most parts of the world. Historically, and in different cultures, a week may have meant 5, 6, 8, 9 or even 10 days, and was often defined with reference to regular market days.
  • Wheel of Time – Also known as Kalachakra (Sanskrit), another word for the concept of the Cyclic View of Time.
  • World Line – The complete history of a particular point in space and time, as represented by a line in four-dimensional space-time, a particularly useful idea in relativistic time. The concept of a world line is also used in time management to refer to a sequence of events that includes the time along with place or location information.
  • Year – A unit of measurement of time, roughly based on the orbital period of the Earth as it moves around the Sun. A solar year is equal to approximately 365.242 days, or about 8,766 hours or 31,556,900 seconds. Because a solar year is not equivalent to an exact number of days, calendar years are an approximation of this period in a given calendar. For example, a calendar year in the Gregorian and Julian calendars has 365 days in a regular or common year, and 366 days in the occasionally inserted leap years.
  • Zeitgeber – A German word, literally meaning “time-giver”, that refers to any external or environmental cue that entrains or synchronizes an organism’s circadian clock and circadian rhythms to the Earth’s 24-hour light/dark cycle. Other Zeitgebers, like the timing of meals, ambient temperatures, exercise, etc, are also used by the body’s peripheral biological clocks.