pitch pipe

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

• noun A small pipe that, when sounded, gives the initial pitch for a piece of music or the standard pitch for tuning an instrument.

from The Century Dictionary.

• noun A small musical pipe of wood or metal to be sounded with the breath, by which the proper pitch of a piece of music may be given, or an instrument tuned.

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

• noun A device much like a harmonica, used to supply a sought pitch.

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

• noun a small pipe sounding a tone of standard frequency; used to establish the starting pitch for unaccompanied singing