A watch is a mechanical device used to measure or indicate the passage of time. Unlike a clock it is design to be carried or worn. A watch requires a power source and a mechanical means of transmitting and controlling it, as well as indicators to register the lapse of time.

A watch generally uses a coiled spring as its power source. As in spring-powered clocks, the watch conserves energy by means of a gear train with a balance wheel regulating the motive force. In self-winding watches the mainspring is tightened automatically by means of a weight on a rotor that responds to the arm movements of the wearer. The electric or electronic wristwatch is powered by a tiny battery that functions for one year without replacement.

Precision timepieces known as chronometers are precision devices used by navigators to determine their longitude at sea and by astronomers and jewelers for calibrating measuring devices. The modern wrist chronometer is a precision watch regulated in different positions and at various temperatures and awarded a certificate by testing bureaus in Switzerland. Another precision timepiece is the chronograph, which not only provides accurate time but also registers elapsed time in fractions of a second. Various forms of chronographs exist. The timer or stopwatch is used in athletic contests and shows elapsed time without providing the time or date.