Definition and background:
Aerophone, Edge-blown Aerophone, Percussion instrument, Auxiliary Percussion; This instrument is a special effect that creates a sound similar to the steam whistle of a boat. The original boat whistle was a steam whistle designed to provide an extremely loud sound to warn nearby ships of an emergency or alert ships to their location in fog. The boat whistle is a hand-held mouth-blown instrument that can be made out of metal, wood, or plastic and often made of chrome. They are created with multiple pipes to sound multiple pitches, typically with two or three pitches. As with the original boat whistles, each boat whistle can have a unique set of tones and a unique sound quality. The pipes of the boat whistle are normally created to produce pitches that are dissonant. The boat whistle has no way of easily altering or adjusting pitch, so there is no range, nor is there any specific pitch or set of pitches associated with the instrument.
For more information on the music term "Boat whistle" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary