What is time? A complete guide to the definition of time

Time is the ordering of events and the measure of how long they last: the continuing sequence from past through present to future. Science defines it through the second, 9,192,631,770 vibrations of a cesium atom. But time is also something we feel, remember and plan, and every culture organizes it with clocks, calendars and celebrations. This guide covers all of it.

Everyday meanings of "time"

In ordinary speech, "time" does several jobs. It can be a point ("the meeting starts at three"), a duration ("the drive takes two hours"), an era ("in ancient times"), an occasion ("the first time") or an opportunity ("now is the time"). The dictionary lists these meanings:

One useful distinction runs through everything below: a moment is a position in time (9:00 AM), while a duration is the interval between two moments (9:00 to 10:30 is 1 hour 30 minutes). Clocks show moments; stopwatches and timers measure durations.

How time is measured

Every clock ever made works the same way: it counts something that repeats regularly. Early people counted days, moons and seasons. Sundials, water clocks and hourglasses divided the day. Mechanical clocks counted the swings of a balance or, after Christiaan Huygens' 1656 pendulum clock, a pendulum, quartz watches count the vibrations of a crystal, and atomic clocks count the frequency of cesium atoms, which never varies.

The second, minute and hour fit together exactly; we have 24 hours and 60 minutes thanks to the Egyptians and Babylonians. Months and years don't fit neatly into days, which is why calendars need rules: the Gregorian calendar adds a leap day most fourth years to stay in step with the seasons.

Measurement also needs agreement. The world's clocks follow Coordinated Universal Time (UTC), averaged from hundreds of atomic clocks, and local clocks are set to a time zone offset from it. A clock is an instrument that displays time; it isn't time itself. That's why two clocks can disagree: one may simply be set wrong. Check yours against the exact time.

Time in physics: from Newton to Einstein

Newton's absolute time. In 1687 Isaac Newton described "absolute, true and mathematical time" that flows evenly everywhere, regardless of what happens in it. For everyday life this is an excellent approximation: a clock in your kitchen and one in Tokyo tick at the same rate.

Einstein's relative time. In 1905 Albert Einstein showed that time depends on motion: a moving clock runs slow compared with one at rest, and observers moving differently can disagree about whether two distant events happened at the same moment. In 1915 he added gravity: clocks closer to a large mass run slower. Space and time are joined into a single four-dimensional spacetime. These effects are real and measurable. GPS satellites gain about 38 microseconds a day relative to ground clocks and must be corrected for it. Read more in Einstein, relativity and time dilation.

Quantum time. In quantum mechanics, time is usually treated as an outside parameter rather than something you measure, and attempts to combine quantum theory with gravity raise the possibility that time isn't fundamental at all but emerges from something deeper. This is one of the biggest open questions in physics.

The arrow of time: why time goes forward

Most laws of physics work equally well run backward, yet the world clearly has a direction: a glass shatters but never reassembles, coffee cools but never heats up on its own. In 1927 the astronomer Arthur Eddington called this "time's arrow". The usual explanation is the second law of thermodynamics: in an isolated system, entropy, roughly the number of ways things can be arranged, tends to increase. The universe began in a very low-entropy state, and everything since has been running "downhill". Memory follows the same arrow: records of events (in brains, photos and documents) only exist for the past, which is why we remember yesterday but not tomorrow. Physicists sometimes speak of three arrows that all point the same way: the thermodynamic arrow (entropy rises), the cosmological arrow (the universe expands) and the psychological arrow (we remember the past, not the future).

What philosophers say about time

Augustine, around 400 AD, put the problem best: "What then is time? If no one asks me, I know; if I wish to explain it to one who asks, I know not." He noted that the past no longer exists and the future doesn't exist yet, and concluded that we hold time in the mind as memory, attention and expectation.

Kant argued in the 1780s that time is a form the mind uses to organize experience: we can't perceive anything except as happening in sequence.

Presentism vs eternalism. Presentists hold that only the present moment is real. Eternalists hold that past, present and future are all equally real, like places on a map, which some call the "block universe". A middle view, the growing block, says the past and present are real but the future isn't yet, and the block grows as time passes. Relativity, with no single universal "now", is often taken to support eternalism. Einstein himself wrote, consoling a friend's family in 1955, that the distinction between past, present and future is "only a stubbornly persistent illusion".

McTaggart (1908) argued that time is unreal. Few accept his conclusion, but his split between tensed time (past, present, future) and tenseless time (earlier and later) still frames the debate. See our longer essay, Is time real?

Time in the mind

Clock time is even; experienced time is not. Ten minutes waiting for news can feel like an hour, while an absorbing afternoon disappears. The brain has no single clock; different systems handle the timing of movements, short intervals and the order of life events. Attention, emotion and novelty all stretch or shrink how long things feel.

Two effects stand out. First, time seems to speed up with age: each year is a smaller share of a longer life, and familiar routines leave fewer distinct memories, so years seem shorter looking back. Second, our body keeps its own roughly 24-hour circadian rhythm, run by a master clock in the brain called the suprachiasmatic nucleus (SCN), which resets each day from the light reaching our eyes. It times sleep, body temperature and hormones, which is why sleep timing and jet lag matter.

Time in culture: calendars, cycles and sacred days

Every society has needed to agree on when to plant, trade, rest and celebrate. Calendars turn the cycles of the Sun and Moon into a shared rhythm: solar calendars (Gregorian) follow the seasons, lunar calendars (Islamic) follow the Moon, and lunisolar calendars (Chinese, Hebrew, Hindu) combine both by adding an extra month every few years.

Many traditions see time as cyclical, built on returning days, seasons and festivals, while others emphasize a linear story with a beginning and an end. Most cultures mix the two. Anthropologist Edward T. Hall added another split: monochronic cultures treat time as a line of separate slots, valuing schedules, punctuality and one thing at a time, while polychronic cultures treat it as flexible, doing several things at once and putting people ahead of the timetable. Being 15 minutes late means very different things in Zurich and in Rio. Communities everywhere mark the same turning points of the year: the solstices and equinoxes, the new year, the harvest, and festivals of light in the dark of winter. See sacred times of the year around the world.

Clocks changed how people live. Monasteries rang bells for prayer hours, towns built clock towers, factories paid by the hour, and in 1883–84 railways and astronomers created standard time and time zones. Benjamin Franklin's "time is money" (1748) captures the change. Today, phones put the whole world on the same clock, and many people feel more pressed for time than ever.

The word itself varies: tiempo, Zeit, время, 時間, waqt. See "time" in other languages and more words about time.

Time and a human life

Time is the one resource everyone receives at the same rate and nobody can buy back. An 80-year life is about 4,170 weeks. That limit is what gives choices weight; see why time is the most important thing we have. The Roman philosopher Seneca put it this way: "It is not that we have a short time to live, but that we waste a lot of it."

The Greeks had two words that help here. Chronos is measured time, the hours on the clock. Kairos is the right moment, the time when something should happen. Good time management needs both: a plan for the hours, and attention to what matters now.

Putting it together: a working definition

No single field captures everything time is. Physics describes how events are ordered and how clocks behave. Psychology describes how we experience duration. Calendars and clocks are the social systems that coordinate us. And for each person, time is the limited span in which a life happens. Combined:

Time is the measurable and experienced ordering of change: a physical relation between events, kept by clocks; a structure of memory and expectation in the mind; a shared system of calendars and schedules; and the limited span in which every life unfolds.

Sources

• BIPM: SI definition of the second
• Stanford Encyclopedia of Philosophy: Time
• Internet Encyclopedia of Philosophy: Time
• Wikipedia: Time
• Online Etymology Dictionary: time

Last updated October 18, 2026.