Time vocabulary: the A to Z of time

The A to Z of time: 379 time words and terms, each with a short, checked definition. 175 everyday words (afternoon, fortnight, o'clock, Zulu time) for students and English learners, plus 204 advanced terms from physics, astronomy, computing, biology and business, from attoseconds to the heat death of the universe. Search, filter by topic or level, or download the printable everyday word list (PDF).

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absolute time Science advanced: Newton's idea of a single universal time flowing the same for everyone; relativity showed it isn't so.
accuracy vs precision Science advanced: Accuracy is how close a clock is to the true time; precision (stability) is how consistently it ticks.
after Words: Later than something else: "after lunch".
afternoon Day: The part of the day from noon until evening.
ago Words: Before now: "two hours ago".
alarm clock Clocks: A clock that makes a sound at a time you set, to wake you up.
AM Day: The hours from midnight to just before noon. Short for ante meridiem, Latin for "before midday".
anachronism Words advanced: Something placed in the wrong time period, like a wristwatch in a movie about ancient Rome.
analog clock Clocks: A clock with a round face and moving hands.
anniversary Calendar: The same date in a later year as an important event.
annual Calendar: Happening once a year.
anomalistic year Astronomy advanced: From one closest approach to the Sun (perihelion) to the next, about 365.2596 days.
Anthropocene Science advanced: A proposed name for the era of human impact on Earth; geologists voted in 2024 not to make it an official epoch.
aphelion Astronomy advanced: Earth's farthest point from the Sun, in early July.
apparent solar time Astronomy advanced: Time by the actual Sun in the sky; a sundial shows it.
arrow of time Science: The one-way direction of time from past to future; physicists link it to entropy, which always tends to increase.
astronomical clock Clocks advanced: A clock that also shows the Sun, Moon, zodiac or planets, like the Prague Orloj from 1410.
astronomical twilight Astronomy advanced: Sun 12–18° below the horizon; after it ends, the sky is fully dark.
atomic clock Clocks: The most accurate kind of clock; it counts the vibrations of atoms such as cesium.
attosecond Units advanced: 10⁻¹⁸ seconds; used to watch electrons move. Attosecond pulses won the 2023 Nobel Prize in Physics.
autumn Seasons: The season between summer and winter; also called fall.
autumnal equinox Seasons: The day in September (Northern Hemisphere) when day and night are about equal and autumn begins.
balance wheel Clocks advanced: The spinning, spring-driven wheel that regulates a mechanical watch.
BC / AD Calendar advanced: Before Christ and Anno Domini (“in the year of the Lord”); there is no year 0 between 1 BC and AD 1.
BCE / CE Calendar advanced: Before Common Era and Common Era: the same years as BC and AD.
bedtime Day: The time you usually go to bed.
before Words: Earlier than something else: "before school".
belated Words: Late; coming after the right time: "a belated birthday card".
bell Clocks: Many old clocks strike a bell to sound the hours; the word clock comes from the Latin for bell.
biannual Calendar: Twice a year.
bicentennial Calendar: A 200th anniversary.
biennial Calendar: Once every two years.
Big Bang Science: The start of the universe as we know it, about 13.8 billion years ago; time as we measure it begins there.
Big Ben Clocks advanced: The nickname of the great bell in the Elizabeth Tower clock in London, often used for the clock itself.
billable hours Business advanced: Hours of work charged to a client.
biological clock Science: The body's internal timekeeper, which controls sleep, hunger and hormones on a roughly 24-hour cycle.
block time Words: In air travel, the gate-to-gate time of a flight.
block universe Science advanced: The view, inspired by relativity, that past, present and future all exist together in four-dimensional spacetime.
blue hour Astronomy advanced: The deep-blue twilight just before sunrise or after sunset.
business day Business advanced: A working day, usually Monday to Friday excluding public holidays.
calendar Calendar: A chart or system for organizing days, weeks, months and years.
calendar year Calendar: January 1 to December 31.
carbon dating Science advanced: Radiocarbon dating: using carbon-14 (half-life about 5,730 years) to date once-living material up to about 50,000 years old.
carpe diem Words advanced: Latin for “seize the day”.
causality Science advanced: The principle that causes come before their effects; nothing can influence the past.
century Calendar: 100 years.
cesium-133 Science advanced: The atom used to define the second: exactly 9,192,631,770 cycles of its radiation equal one second.
chronobiology Science: The science of biological rhythms, such as sleep cycles and jet lag.
chronograph Clocks advanced: A watch with a built-in stopwatch.
chronological Words: In the order things happened, from first to last.
chronology Words: A list of events in the order they happened.
chronometer Clocks: A very accurate clock, especially one used on ships to find longitude.
chronos Words advanced: Greek for sequential clock time; also the personification of time.
chronos and kairos Words: Two ancient Greek words for time: chronos is clock time; kairos is the right or opportune moment.
chronostasis Science: The "stopped clock" illusion: when you first glance at a clock, the second hand seems to pause longer than a second.
chronotype Biology advanced: Your natural tendency to be a morning person (“lark”) or night person (“owl”).
circadian rhythm Science: The roughly 24-hour cycle of the body clock, set mainly by daylight.
civil time Calendar advanced: The official clock time people use in a place, set by law.
civil twilight Astronomy advanced: When the Sun is 0–6° below the horizon; usually enough light to see outdoors.
clepsydra Clocks advanced: The ancient Greek name for a water clock.
clock Clocks: A device that measures and shows the time.
clock drift Clocks advanced: A clock's gradual gain or loss of time compared with a reference.
clock face Clocks: The front of a clock, with numbers or marks around the edge; also called the dial.
complication Clocks advanced: Any watch function beyond hours and minutes, like a date or moon phase.
Coordinated Universal Time (UTC) Science: The world's standard time, kept by atomic clocks. Time zones are written as UTC plus or minus hours.
cosmic time Science advanced: The time since the Big Bang measured by an observer moving with the expansion of the universe; about 13.8 billion years now.
countdown Words: Counting down the time left until something happens.
countdown clock Words advanced: A clock counting down to an event, like a rocket launch or New Year.
critical path Business advanced: The longest chain of dependent tasks in a project; it sets the shortest possible finish time.
cron Computing advanced: A Unix scheduler that runs tasks at set times, like every day at 03:00.
cuckoo clock Clocks: A clock with a little bird that pops out and calls "cuckoo" on the hour.
date Calendar: A particular day, such as March 14, 2027.
dawn Day: The first light of the morning, before sunrise.
day Calendar: 24 hours; the time Earth takes to spin once (relative to the Sun).
day of the year Calendar advanced: A day's number counting from January 1 (1 to 365 or 366).
daylight Day: The natural light of day, between sunrise and sunset.
daylight saving time Calendar: Setting clocks one hour ahead in summer for lighter evenings. Often called "daylight savings".
days of the week Calendar: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.
daytime Day: The hours of daylight.
dead reckoning Science advanced: Estimating position from known speed, direction and elapsed time.
deadline Words: The latest time something must be finished.
decade Calendar: 10 years.
deep time Science: The enormous span of Earth's history, about 4.5 billion years.
delay Words: Something that makes you late, or to make something happen later.
dial Clocks: The face of a clock or watch.
digital clock Clocks: A clock that shows the time in numbers, like 10:45.
doomsday clock Words advanced: A symbolic clock set by the Bulletin of the Atomic Scientists to show how close humanity is to global catastrophe.
duration Words: How long something lasts.
dusk Day: The darkest part of twilight, just before night.
early Words: Before the expected time.
eclipse Astronomy advanced: When one body blocks the light of another; eclipse timings were among the first precise predictions in astronomy.
entropy Science: A measure of disorder; it tends to increase over time, which is one way physicists explain why time only moves forward.
eon Science: A very long time; in geology, the largest division of Earth's history.
ephemeral Words advanced: Lasting a very short time.
ephemeris Astronomy advanced: A table of where the Sun, Moon, planets or satellites will be at given times.
epoch Science: A point in time from which time is counted, such as January 1, 1970 for computers.
equation of time Astronomy advanced: The difference between sundial time and clock time, up to about 16 minutes depending on the date.
equinox Seasons: One of two days a year (March and September) when day and night are about the same length.
era Calendar: A long period of history, often counted from an important event.
escapement Clocks advanced: The mechanism that releases a clock's energy in equal ticks; the heart of every mechanical clock.
eternalism Science advanced: The philosophical view that past, present and future are equally real.
evening Day: The end of the day, from about sunset until bedtime.
event horizon Science advanced: The boundary around a black hole beyond which nothing, not even light, can get back out.
fall Seasons: The season between summer and winter; also called autumn. Also "fall back", when clocks go back an hour.
femtosecond Units advanced: 10⁻¹⁵ seconds; the timescale of chemical reactions.
fiscal year Calendar: A 12-month period a business or government uses for its accounts; it may not start in January.
fortnight Calendar: Two weeks (14 days).
frame rate Computing advanced: Frames shown per second (fps): 24 for film, 30 or 60 for video.
frequency Science advanced: How many times something repeats per second, measured in hertz (Hz). Clocks count a steady frequency.
frequency comb Science advanced: A laser tool that links optical and microwave frequencies, making optical clocks usable; won the 2005 Nobel Prize.
full-time equivalent (FTE) Business advanced: A unit equal to one full-time worker's hours, often 40 a week.
future Words: The time that has not happened yet.
galactic year Science: The time the Sun takes to travel once around the center of the Milky Way, roughly 225 to 250 million years.
galactic year (cosmic year) Astronomy advanced: About 225 to 250 million years for the Sun to orbit the Milky Way.
Gantt chart Business advanced: A bar chart showing a project's tasks along a timeline.
generation Calendar: People born around the same time, or about 25 to 30 years.
generation time Biology advanced: The average time between one generation and the next.
geologic time Science: The huge time scale of Earth's history, measured in millions and billions of years.
geologic time scale Science advanced: Earth's history divided into eons, eras, periods and epochs.
gestation period Biology advanced: How long an animal develops before birth: about 40 weeks for humans, about 22 months for elephants.
gigasecond Units advanced: 1 billion seconds, about 31.7 years.
gigayear (Gyr) Units advanced: One billion years. Earth is about 4.54 Gyr old.
gnomon Clocks: The part of a sundial that casts the shadow.
golden hour Astronomy advanced: The period shortly after sunrise or before sunset when light is soft and warm.
GPS time Science: The time kept by GPS satellites; 18 seconds ahead of UTC.
grace period Business advanced: Extra time allowed after a deadline before a penalty applies.
grandfather clock Clocks: A tall floor clock with a swinging pendulum.
gravitational time dilation Science advanced: Clocks run slower deeper in gravity: a clock on the ground ticks slightly slower than one on a mountain.
Greenwich Mean Time (GMT) Science: The time at Greenwich, London, once the world's standard; today almost the same as UTC.
Gregorian calendar Calendar advanced: The calendar most of the world uses, introduced by Pope Gregory XIII in 1582.
half-hour time zone Calendar advanced: A zone offset by 30 minutes, like India (UTC+5:30); Nepal even uses UTC+5:45.
half-life Science: The time for half of a radioactive substance to decay; used to date ancient objects.
hands Clocks: The pointers on a clock: the hour hand, minute hand and second hand.
heat death Science advanced: A possible far-future end state of the universe in which no usable energy is left; a hypothesis, not a certainty.
hertz (Hz) Units advanced: One cycle per second. A quartz watch crystal vibrates 32,768 times a second (32,768 Hz).
high noon Day: Exactly noon; 12 o'clock midday.
hindsight Words advanced: Understanding an event only after it has happened.
holdover Clocks advanced: How long a clock stays accurate after losing its reference signal, such as GPS.
holiday Calendar: A special day of celebration, often a day off.
holiday paradox Biology advanced: A holiday feels fast while it happens but long when remembered, because it's full of new memories.
Holocene Calendar advanced: The current geological epoch, which began about 11,700 years ago after the last ice age.
horology Clocks: The study and making of clocks and watches.
hour Units: 60 minutes. There are 24 hours in a day.
hour hand Clocks: The short hand on a clock; it points to the hour.
hourglass Clocks: Two glass bulbs joined by a narrow neck; sand flows from top to bottom in a set time.
Hubble time Science advanced: One divided by the Hubble constant, roughly 14 billion years; a rough scale for the age of the universe.
hydrogen maser Clocks advanced: An atomic clock with superb short-term stability, used by the Deep Space Network and timing labs.
hyperbolic discounting Biology advanced: Preferring a smaller reward now over a bigger one later, more than logic would suggest.
IANA time zone database Computing advanced: The public database (tzdata) of every time zone's rules and history, used by phones and computers.
infradian rhythm Biology advanced: A body rhythm longer than a day, like a monthly cycle.
intercalation Calendar advanced: Inserting an extra day or month into a calendar, like February 29.
International Date Line Science: An imaginary line in the Pacific where the calendar date changes by one day.
ISO 8601 Computing advanced: The international date-time format, such as 2026-10-22T14:30:00Z.
ISS time Science advanced: The International Space Station runs on UTC, which crews call GMT.
J2000 Astronomy advanced: The standard astronomical reference moment: January 1, 2000, 12:00 Terrestrial Time.
jet lag Words: Feeling tired and out of sync after flying across several time zones.
jiffy Words: Informally, a very short time ("back in a jiffy"). Scientists and engineers have given it exact values, which vary.
jiffy (computing) Computing advanced: In many computer systems, one tick of the system timer, often 1/100 or 1/1000 of a second.
jitter Computing advanced: Small, irregular variation in timing, such as uneven packet arrivals.
jubilee Calendar: A special anniversary, such as a 25th (silver) or 50th (golden).
Julian calendar Calendar advanced: Julius Caesar's calendar from 45 BC, with a leap year every 4 years; some churches still use it.
Julian date Astronomy advanced: A continuous count of days (and fractions) since noon on January 1, 4713 BC, used by astronomers.
Julian year Units advanced: Exactly 365.25 days (31,557,600 seconds), used in astronomy; a light-year is based on it.
just-in-time Business advanced: Making or delivering things exactly when needed, to avoid storing stock.
ka / kyr Units advanced: Thousands of years, as in “12 ka” (12,000 years ago) in geology and archaeology.
kilosecond Units advanced: 1,000 seconds, about 16 minutes 40 seconds.
late Words: After the expected or right time.
latency Computing advanced: The delay between an action and its response, such as a network round trip.
later Words: At a time after now.
lead time Business advanced: The time between ordering something and receiving it.
leap second Science: An extra second sometimes added to UTC to keep clocks in step with Earth's spin.
leap year Calendar: A year with 366 days; February gets a 29th day.
leap year rule Calendar advanced: Every 4 years, except century years unless divisible by 400: 2000 was a leap year, 1900 wasn't.
life expectancy Biology advanced: The average number of years a person is expected to live, based on statistics.
lifespan Biology advanced: How long a living thing lives.
light cone Science advanced: In relativity, the region of spacetime an event can affect (future) or be affected by (past), limited by light speed.
light-time Astronomy advanced: How long light takes to travel a distance: 1.3 s to the Moon, about 8 min 20 s from the Sun.
light-year Science: A distance, not a time: how far light travels in one year, about 9.46 trillion km.
local mean time Astronomy advanced: Mean solar time at a particular longitude; every town used its own before time zones.
Long Now Words advanced: The idea of thinking in 10,000-year spans, behind a planned clock built to run for 10,000 years.
longitude problem Science advanced: Finding a ship's east-west position, solved by John Harrison's marine chronometers in the 1700s.
lunar calendar Calendar advanced: A calendar based on Moon phases, like the Islamic (Hijri) calendar of about 354 days.
lunar month Calendar: The time from one new moon to the next, about 29.5 days.
Lunar time (LTC) Science advanced: Coordinated Lunar Time, a planned time standard for the Moon that NASA has been asked to define by the end of 2026.
lunisolar calendar Calendar advanced: A calendar with lunar months plus extra months to stay in step with the seasons, like the Hebrew and Chinese calendars.
mainspring Clocks advanced: The coiled spring that powers a mechanical clock or watch.
mean solar time Astronomy advanced: Solar time averaged over the year so every day is exactly 24 hours long.
megasecond Units advanced: 1 million seconds, about 11.6 days.
megayear (Myr) Units advanced: One million years; also written Ma (mega-annum) for “million years ago”.
melatonin Biology advanced: The hormone that rises in the evening and signals the body that it's time to sleep.
memento mori Words advanced: Latin for “remember you will die”, a reminder to use time well.
mental chronometry Biology advanced: The study of how long mental processes take.
meridiem Day: Latin for "midday"; the "m" in AM and PM.
metrology Science advanced: The science of measurement, including time and frequency.
microsecond Units: One millionth of a second.
midday Day: 12 o'clock noon.
midnight Day: 12 o'clock at night, when a new day begins.
midnight sun Astronomy advanced: When the Sun stays above the horizon all night in summer near the poles.
military time Clocks: The 24-hour clock written without a colon, like 1430 for 2:30 PM.
millennium Calendar: 1,000 years.
millisecond Units: One thousandth of a second.
minute Units: 60 seconds. There are 60 minutes in an hour.
minute hand Clocks: The long hand on a clock; it points to the minutes.
Mission Elapsed Time (MET) Science advanced: A spaceflight clock that starts at zero at liftoff.
modified Julian date (MJD) Astronomy advanced: The Julian date minus 2,400,000.5, giving smaller numbers starting at midnight.
moment Words: A very short time.
monotonic clock Computing advanced: A computer clock that only ever moves forward, used for measuring durations even if the wall clock is changed.
month Calendar: One of the 12 parts of the year, 28 to 31 days long.
morning Day: The part of the day from sunrise (or midnight) until noon.
MTBF Business advanced: Mean time between failures: the average running time between breakdowns of a repairable system.
nanosecond Units: One billionth of a second; light travels about 30 cm in a nanosecond.
nautical twilight Astronomy advanced: Sun 6–12° below the horizon; the horizon at sea is still visible.
night Day: The dark hours between sunset and sunrise.
Noether's theorem Science advanced: Emmy Noether's 1918 result: because physics laws don't change over time, energy is conserved.
noon Day: 12 o'clock in the middle of the day.
nostalgia Words advanced: A sentimental longing for the past.
now Words: At this moment.
NTP Computing advanced: Network Time Protocol: keeps computer clocks in sync over the internet, typically within milliseconds.
nuclear clock Clocks advanced: An experimental clock using a transition in the thorium-229 nucleus instead of electrons.
o'clock Clocks: Short for "of the clock"; used for exact hours, like 3 o'clock.
oddball effect Biology advanced: A surprising event seems to last longer than an ordinary one of the same length.
Old Style / New Style Calendar advanced: Dates written in the Julian (O.S.) or Gregorian (N.S.) calendar; Britain switched in 1752 and skipped 11 days.
on time Words: At the planned time, not late.
one-way light time Astronomy advanced: The signal travel time between a spacecraft and Earth.
opportunity cost Business advanced: The value of the next-best thing you give up when you spend time or money on something.
optical clock Clocks advanced: A next-generation atomic clock using light frequencies (strontium, ytterbium); far more precise than cesium clocks.
optical lattice clock Clocks advanced: An optical clock that holds thousands of atoms in a grid of laser light.
orbital period Astronomy advanced: The time one body takes to go around another: 365.25 days for Earth, about 90 minutes for the ISS.
oscillator Clocks advanced: Anything that swings or vibrates at a steady rate to keep time: a pendulum, a quartz crystal, an atom.
overtime Words: Extra time worked beyond normal hours, or extra time in a game.
past Words: The time that has already happened. In telling time, "ten past three" means 3:10.
pay period Business advanced: How often you're paid: weekly, biweekly, semimonthly or monthly.
pendulum Clocks: A weight that swings back and forth at a steady rate to keep a clock on time.
per annum Calendar: Latin for "per year".
per diem Calendar: Latin for "per day"; often a daily allowance for expenses.
perennial Words advanced: Lasting through many years, or a plant that lives more than two years.
perihelion Astronomy advanced: Earth's closest point to the Sun, in early January.
period Science advanced: The time one complete cycle takes; the opposite of frequency (period = 1 ÷ frequency).
perpetual calendar Calendar advanced: A calendar or chart that shows the weekday for any date across many years.
picosecond Units: One trillionth of a second.
Planck time Science: About 5.4 × 10⁻⁴⁴ seconds, the shortest interval current physics can meaningfully describe.
planning fallacy Biology advanced: The tendency to underestimate how long tasks will take, even when past tasks ran late.
PM Day: The hours from noon to just before midnight. Short for post meridiem, Latin for "after midday".
pocket watch Clocks: A watch carried in a pocket, often on a chain.
polar night Astronomy advanced: When the Sun doesn't rise for 24 hours or more in polar winter.
power reserve Clocks advanced: How long a fully wound mechanical watch runs before stopping.
precession Astronomy advanced: The slow wobble of Earth's axis over about 26,000 years, which shifts the stars behind the equinoxes.
present Words: The time happening now.
presentism Science advanced: The philosophical view that only the present moment truly exists.
prime meridian Science: The line of 0° longitude through Greenwich, London, from which time zones are measured.
proper time Science advanced: The time measured by a clock traveling along with an object; different paths through spacetime give different proper times.
PTO Business advanced: Paid time off: vacation, sick or personal days that are paid.
PTP (IEEE 1588) Computing advanced: Precision Time Protocol: syncs clocks on a local network to sub-microsecond accuracy.
pulsar Astronomy advanced: A spinning neutron star that sends pulses so regularly that some rival atomic clocks.
pulsar timing Astronomy advanced: Measuring pulsar pulses precisely to detect gravitational waves and test physics.
punctual Words: Always on time.
quantum clock Science advanced: A clock based on quantum transitions in atoms or ions; all atomic clocks are quantum clocks.
quarter (Q1–Q4) Business advanced: A three-month period of the year: Q1 is January to March.
quarter hour Units: 15 minutes. "Quarter past" means 15 minutes after; "quarter to" means 15 minutes before.
quartz clock Clocks: A clock kept accurate by a tiny vibrating quartz crystal; most watches and wall clocks today.
radioactive decay Science advanced: The spontaneous breakdown of unstable atoms at a steady statistical rate, which makes it a natural clock.
radiometric dating Science advanced: Dating rocks and fossils by measuring how much of a radioactive element has decayed.
railway time Calendar advanced: Standard time adopted by railways in the 1800s, which led to time zones.
reaction time Biology advanced: The time between a signal and a response; around a quarter of a second to something you see.
real-time clock (RTC) Computing advanced: A small battery-backed clock chip that keeps time while a device is off.
recently Words: A short time ago.
record time Words: Faster than ever before, or very fast.
regnal year Calendar advanced: Years counted from a monarch's reign, as Japan still does with eras like Reiwa.
resonance Science advanced: The natural frequency at which something vibrates most easily; clocks lock onto a resonance.
rotation period Astronomy advanced: The time a planet takes to spin once; a Mars day (sol) is 24 h 39 min.
rubidium clock Clocks advanced: A compact, low-cost atomic clock used in GPS satellites and cell towers.
Saros cycle Astronomy advanced: A period of about 18 years 11 days after which similar eclipses repeat.
schedule Words: A plan of what will happen and when.
season Seasons: One of the four parts of the year: spring, summer, autumn (fall) and winter.
second Units: The basic unit of time. 60 seconds make a minute; it is defined by atomic clocks.
second hand Clocks: The thin hand on a clock that goes around once a minute.
semester Calendar: Half of a school year.
shake Units advanced: Nuclear physics slang for 10 nanoseconds.
shift Words: A set period of work, such as the night shift.
sidereal day Astronomy advanced: One rotation of Earth relative to the stars: about 23 hours 56 minutes 4 seconds.
sidereal month Astronomy advanced: One orbit of the Moon relative to the stars, about 27.32 days.
sidereal time Science: Time measured by the stars instead of the Sun; a sidereal day is about 4 minutes shorter.
sidereal year Astronomy advanced: One orbit of Earth relative to the stars, about 365.2564 days, 20 minutes longer than the tropical year.
simultaneity Science advanced: Whether two events happen at the same time; in relativity, distant observers moving differently can disagree.
SLA / uptime target Business advanced: A promised level of service, often stated as uptime, like 99.9% (about 8.8 hours of downtime a year).
sleep cycle Biology advanced: A cycle of light, deep and REM sleep lasting about 90 minutes.
slow motion Words advanced: Video recorded at a high frame rate and played back slowly.
social jet lag Biology advanced: The mismatch between your body clock and social schedules, such as sleeping in late on weekends.
sol Astronomy advanced: A Mars day: 24 hours 39 minutes 35 seconds.
solar day Astronomy advanced: From one solar noon to the next, 24 hours on average.
solar noon Astronomy advanced: The moment the Sun is highest in the sky; rarely exactly 12:00 on the clock.
solstice Seasons: The longest or shortest day of the year, in June and December.
someday Words: At some time in the future.
soon Words: In a short time from now.
spacetime Science: Einstein's idea that space and time form a single four-dimensional whole.
spring Seasons: The season between winter and summer. Also "spring forward", when clocks go ahead an hour.
standard time Calendar: The normal time in a time zone when daylight saving time is not in use.
stopwatch Clocks: A timer you start and stop to measure exactly how long something takes.
stratum Computing advanced: An NTP server's distance from a reference clock: stratum 0 is the atomic or GPS clock itself.
summer Seasons: The warmest season, between spring and autumn.
summer solstice Seasons: The longest day of the year: around June 21 in the Northern Hemisphere.
sundial Clocks: An ancient clock that tells the time by the shadow of the Sun.
sunrise Day: When the Sun appears above the horizon in the morning.
sunset Day: When the Sun disappears below the horizon in the evening.
suprachiasmatic nucleus Biology advanced: A tiny region of the brain that acts as the body's master clock.
synchronize Words: To make clocks or events match exactly in time.
synodic month Astronomy advanced: New moon to new moon, about 29.53 days.
system clock Computing advanced: The operating system's idea of the current time.
TAI Science advanced: International Atomic Time: a continuous scale from about 450 atomic clocks worldwide; currently 37 s ahead of UTC.
tardy Words: Late, especially for school.
TDB / TCB Science advanced: Barycentric time scales used to calculate planet and spacecraft motion across the solar system.
tempo Words: The speed of music, counted in beats per minute.
temporal Words advanced: Relating to time (or to worldly rather than spiritual things).
tempus fugit Words advanced: Latin for “time flies”, seen on many old clocks.
Terrestrial Time (TT) Science advanced: The time scale for astronomical calculations seen from Earth's surface: TAI + 32.184 s.
tidal braking Astronomy advanced: The Moon's tides slowly slow Earth's spin, lengthening the day by roughly 2 milliseconds per century.
tidal locking Astronomy advanced: When a moon's rotation matches its orbit so it always shows the same face, as our Moon does.
time Words: The ongoing passage of events from past to present to future, measured in seconds, minutes, hours and more.
time ball Clocks advanced: A large ball dropped from a mast at a set time so ships could set their chronometers; Greenwich's dates from 1833.
time blindness Biology advanced: Difficulty sensing how much time has passed or how long tasks will take.
time capsule Words advanced: A container of objects sealed to be opened in the future.
time complexity Computing advanced: How an algorithm's running time grows with the size of its input, written like O(n).
time dilation Science: Time passing at different rates for clocks moving fast or in different gravity, as Einstein predicted; GPS must correct for it.
time perception Science: How fast time feels to pass; it speeds up when you're busy and seems to drag when you're bored or waiting.
time signature Words advanced: In music, the notation (like 4/4) showing beats per bar.
time symmetry Science advanced: Most basic physics laws work the same forwards and backwards in time; the arrow of time comes mainly from entropy.
time transfer Science advanced: Comparing or sharing time between distant clocks, by satellite, radio or fiber.
time travel Science advanced: Traveling forward in time faster than normal is possible via relativity (in tiny amounts); traveling to the past remains speculative.
time value of money Business advanced: Money now is worth more than the same amount later, because it can earn interest.
time zone Calendar: A region that uses the same clock time.
time-lapse Words advanced: Photos taken at intervals and played fast to show slow change.
timecode Computing advanced: Hours:minutes:seconds:frames labels used to sync video and audio.
timeline Words: Events shown in the order they happened, often on a line.
timely Words: Happening at the right time.
timeout Computing advanced: A limit after which a system stops waiting for a response.
timepiece Clocks: Any clock or watch.
timer Clocks: A device that counts down from a set time and signals when it reaches zero.
timesheet Words: A record of the hours someone worked.
timestamp Computing advanced: A recorded date and time attached to an event, file or message.
timetable Words: A list of times when buses, trains, classes or events happen.
today Calendar: This day.
tomorrow Calendar: The day after today.
tonight Day: This evening or night.
tourbillon Clocks advanced: A rotating cage for a watch's escapement, invented by Breguet in 1801 to even out gravity's effect.
transit Astronomy advanced: When a body crosses the meridian, or passes in front of another (like Mercury crossing the Sun).
trimester Calendar: A period of about three months, such as a third of a school year or of a pregnancy.
tropical year Astronomy advanced: From one March equinox to the next, about 365.2422 days; calendars follow it to keep seasons in place.
turnaround time Business advanced: The time from submitting a task to getting the finished result.
twilight Day: The soft light just after sunset or just before sunrise.
twin paradox Science advanced: A thought experiment: a twin who travels near light speed returns younger than the twin who stayed home.
ultradian rhythm Biology advanced: A body rhythm repeating more than once a day, like the roughly 90-minute sleep cycle.
Unix epoch Computing advanced: 00:00:00 UTC on January 1, 1970, the starting point of Unix time.
Unix time Science: The number of seconds since January 1, 1970, used by computers.
uptime Computing advanced: How long a computer or service has been running without a restart.
UT1 Science advanced: Universal Time based on Earth's actual rotation; UTC is kept within 0.9 s of it.
UTC offset Calendar advanced: How many hours a time zone is ahead of or behind UTC, like UTC−5 for New York in winter.
vernal equinox Seasons: The day in March (Northern Hemisphere) when spring begins and day and night are about equal.
watch Clocks: A small clock worn on the wrist or carried in a pocket.
water clock Clocks: An ancient clock that measured time by the steady flow of water.
week Calendar: Seven days.
week number Calendar advanced: A week's position in the year; under ISO 8601, week 1 contains the year's first Thursday.
weekday Calendar: Monday to Friday; any day that is not on the weekend.
weekend Calendar: Saturday and Sunday.
winter Seasons: The coldest season, between autumn and spring.
winter solstice Seasons: The shortest day of the year: around December 21 in the Northern Hemisphere.
wristwatch Clocks: A watch worn on a strap around the wrist.
Y2K Computing advanced: The year-2000 bug: software storing years as two digits could read 2000 as 1900.
year Calendar: 365 days (366 in a leap year); the time Earth takes to orbit the Sun.
Year 2038 problem Computing advanced: Older systems storing Unix time in 32 bits will overflow on January 19, 2038, at 03:14:07 UTC.
year-over-year (YoY) Business advanced: Compared with the same period last year.
year-to-date (YTD) Business advanced: From the start of the year until today.
yesterday Calendar: The day before today.
yesteryear Words: The past; years gone by.
Z suffix Computing advanced: A Z at the end of a time (like 14:30Z) means UTC, or “Zulu”.
zeitgeber Biology advanced: German for “time giver”: a cue like daylight or meals that resets your body clock.
Zeno's paradoxes Science advanced: Ancient Greek puzzles (like Achilles and the tortoise) about whether motion through endlessly divisible time is possible.
Zulu time Science: Another name for UTC, used by pilots and the military.
12-hour clock Clocks: Telling time from 1 to 12 twice a day, with AM and PM.
24-hour clock Clocks: Telling time from 00:00 to 23:59 without AM or PM.

Units of time, smallest to largest

Unit	Length	Example
Planck time	about 5.4 × 10⁻⁴⁴ s	Shortest interval current physics can describe
attosecond	10⁻¹⁸ s	Electrons moving inside atoms
femtosecond	10⁻¹⁵ s	Chemical bonds breaking
picosecond	10⁻¹² s	Light travels 0.3 mm
nanosecond	10⁻⁹ s	Light travels about 30 cm
microsecond	10⁻⁶ s	GPS clock correction per day: 38 μs
millisecond	0.001 s	A camera flash; a bee's wingbeat is about 5 ms
second	9,192,631,770 cesium cycles	About one heartbeat
minute	60 s
kilosecond	1,000 s	About 16 min 40 s
hour	3,600 s
day	86,400 s	One rotation of Earth relative to the Sun
week	7 days
fortnight	14 days
megasecond	1,000,000 s	About 11.6 days
month	28–31 days	Lunar month: 29.53 days
year	365 or 366 days	Julian year: exactly 365.25 days
decade	10 years
gigasecond	1 billion s	About 31.7 years
century	100 years
millennium	1,000 years
megayear (Myr)	1 million years	Modern humans: about 0.3 Myr
galactic year	about 225–250 million years	One Sun orbit of the Milky Way
gigayear (Gyr)	1 billion years	Age of the universe: about 13.8 Gyr

Convert any of these with the seconds converter.

The world's time standards

Standard	What it is	Relation to UTC
UTC	Coordinated Universal Time, the world's civil time	reference
TAI	International Atomic Time, averaged from about 450 atomic clocks; no leap seconds	UTC + 37 s
GPS time	Time kept by the GPS satellites since January 6, 1980	UTC + 18 s
TT	Terrestrial Time, used for astronomical predictions	TAI + 32.184 s (UTC + 69.184 s)
UT1	Time by Earth's actual rotation	within 0.9 s of UTC
GMT	Greenwich Mean Time; UK winter civil time, often used as another name for UTC	= UTC (to within a second)
Unix time	Seconds since January 1, 1970, ignoring leap seconds	counts UTC
Julian date	Days since noon, January 1, 4713 BC, used in astronomy	a day count

Live versions: UTC (Zulu) · GPS time and TAI · Unix time · Julian date · sidereal time

Teaching ideas

Clock words first: start with hour hand, minute hand, o'clock, half past and quarter to on the learn to tell time page.
Sort the words: have students group words into "parts of the day", "calendar" and "units"; the topic buttons above show one answer.
Biggest to smallest: put millennium, century, decade, year, month, fortnight, week, day, hour, minute, second, millisecond in order.
Spot the trap: it's ante meridiem and post meridiem (not "meridian"), and the official name is daylight saving time.

More: time idioms and phrases · the word "time" in other languages · adverbs of time

Questions people ask

What are some words about time?

Common time words include second, minute, hour, day, week, month, year, decade, century, morning, afternoon, evening, midnight, noon, early, late, soon, yesterday, today and tomorrow. The full list above has 379, with definitions.

What are all the units of time?

From smallest to largest: Planck time, attosecond, femtosecond, picosecond, nanosecond, microsecond, millisecond, second, minute, hour, day, week, fortnight, month, year, decade, century, millennium, megayear and gigayear. The table above gives each length.

What is the difference between UTC, GMT and TAI?

UTC is the world's civil time standard. GMT is the UK's winter time and is often used to mean UTC. TAI is pure atomic time without leap seconds, currently 37 seconds ahead of UTC.

What does AM and PM stand for?

AM is ante meridiem, Latin for "before midday"; PM is post meridiem, "after midday".

What is a word for two weeks?

A fortnight.

What is the smallest unit of time?

In everyday use, the second; science uses milliseconds, microseconds, nanoseconds and even smaller units. The second itself is defined by atomic clocks.

Is there a printable version?

Yes: download the free PDF of the 175 everyday words, with a classroom activity on the second page.

Sources

Last updated October 22, 2026. Found an error? Tell us.