Definition and background:
A technique employed by woodwind players ( flutes in particular) in which the player directs the flow of air to obtain the pitch of the first overtone or harmonic rather than the fundamental pitch which would normally be sounded.
For more information on the music term "Overblowing" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary