Time has long been a major subject of philosophy, art, poetry, and science. There are widely divergent views about its meaning, hence it is difficult to provide an uncontroversial definition of time. Many fields use an operational definition in which the units of time are defined. Scholars disagree on whether time itself can be measured or is itself part of the measuring system.

The Oxford English Dictionary defines time as "the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole." Another standard dictionary definition is "a nonspatial linear continuum wherein events occur in an apparently irreversible order." The Latin word for time, tempus came from Greek temnein meaning "to cut" (same root for atomos άτομον meaning "indivisible"; thus signifying a division of the flowing duration.

The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum.