Definition and background:
A mechanical device used to provide a stream of air to a musical instrument producing sound (vibrations).A pneumatic device that pumps air into a pipe organ. The pleated center section of instruments in the accordion family that collects air when expanded and pushes the air across the reeds to produce the musical pitch(es) when squeezed together.
For more information on the music term "Bellows" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary