: a device (such as a clock) that indicates by a sound the end of an interval of time or that starts or stops a device at predetermined times. He was the official timer for the race. She set the timer …

us / ˈtɑɪ·mər / Add to word list a device that starts or stops something at a set time or that makes a sound after a particular amount of time has passed:

A timer or countdown timer is a type of clock that starts from a specified time duration and stops upon reaching 00:00. It can also usually be stopped manually before the whole duration has …

a device for controlling machinery, appliances, or the like, in a specified way at a predetermined time: times. Please put the roast in the oven and set the timer to cook it for two hours.

a switch or regulator that causes a mechanism to operate at a specific time or at predetermined intervals

(often in compounds) a device that is used to measure the time that something takes; a device that starts or stops a machine working at a particular time. Once started, the timer runs for one …

There are nine meanings listed in OED's entry for the noun timer, two of which are labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.