tuning fork

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

• noun A small two-pronged metal device that when struck produces a sound of fixed pitch that is used as a reference, as in tuning musical instruments.

from The Century Dictionary.

• noun A steel instrument with two prongs, designed to produce, when struck, a musical tone of some particular pitch.

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

• noun a fork-shaped object which emits a tone of a specific frequency when struck.

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

• noun a metal implement with two prongs that gives a fixed tone when struck; used to tune musical instruments