pitch pipe

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. 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. It is either a flue- or a reed-pipe, and may give either a fixed tone, as A or C, or one of several tones. In the latter case the variation is produced either by a movable plug or stopper altering the length of the air-column, or by a spring that alters the free length of the tongue of the reed.

Wiktionary

1. n. A device much like a harmonica, used to supply a sought pitch.

GNU Webster's 1913

1. n. a wind instrument used by choristers in regulating the pitch of a tune.

WordNet 3.0

1. n. a small pipe sounding a tone of standard frequency; used to establish the starting pitch for unaccompanied singing