timer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who keeps track of time; a timekeeper.
• noun A timepiece, especially one used for measuring and signaling the end of time intervals, as on a stove.
• noun A switch or regulator that controls or activates and deactivates another mechanism at set times.
• noun A device that controls the timing of the sparks that ignite the fuel in an internal-combustion engine.

from The Century Dictionary.

• noun One who keeps or measures and records time; a timekeeper.
• noun A form of stop-watch for recording or indicating short intervals of time.

from the GNU version of the Collaborative International Dictionary of English.

• noun A timekeeper; especially, a watch by which small intervals of time can be measured; a kind of stop watch. It is used for timing the speed of horses, machinery, etc.
• noun (Internal-combustion Engines) In electric ignition, an adjustable device for automatically timing the spark.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Someone or something which times.
• noun A device used to measure amounts of time.
• noun Any electronic function that causes a device to be able to do something automatically after a preset amount of time.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a regulator that activates or deactivates a mechanism at set times
• noun a timepiece that measures a time interval and signals its end
• noun (sports) an official who keeps track of the time elapsed