means A device or function that measures a set interval of time, counting down or up until it signals that the time is up.

from Built from "time" plus the agent suffix "-er," the maker-of-the-thing ending seen in "baker" and "runner." "Time" itself is ancient Germanic — Old English "tīma," tied to a root meaning a stretch or division of duration, a cousin of "tide" back when a tide simply meant a season or fixed period. So a "timer" is, quite literally, a "thing that times."