A clock (from the Latin cloca, "bell") is an instrument for measuring time and for measuring time intervals of less than a day—as opposed to a calendar. Those used for technical purposes, of very high accuracy, are usually called chronometers. A portable clock is called a watch. The clock in its most common modern form (in use since at least the 14th century) displays the hours, minutes and, sometimes, seconds that pass during a twelve- or twenty-four-hour period.

History

The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes. The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely known in ancient times.