a timepiece that measures a time interval and signals its end
"She set a kitchen timer to signal when the cookies were done baking."
(sports) an official who keeps track of the time elapsed
"The referee acted as a timer, signaling when each runner completed their lap in the race."
a regulator that activates or deactivates a mechanism at set times
"The kitchen timer deactivated the oven exactly when the bread was finished baking."
Someone or something which times.
"The automated timer triggered the sprinklers every twenty minutes to water the garden."
In plain English: A timer is a device that counts down from a set amount of time and then makes noise to let you know when it's finished.
"She set the timer for twenty minutes so she wouldn't burn the toast."
The word timer is formed by adding the suffix -er to the verb time. It was first used to describe a person or device that measures duration.