alarm clock

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

• noun A clock that can be set to wake someone up or alert attention at a particular time, as by sounding a bell or buzzer.

from The Century Dictionary.

• noun A clock which can be so set as to make a loud and continued noise at a particular time, in order to arouse from sleep or attract attention.

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

• noun A clock with an alarm that can be set to sound at a determined time.

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

• noun a clock that wakes a sleeper at some preset time