The American Heritage® Dictionary of the English Language, 4th Edition
n. A whistlelike mouthpiece for certain wind instruments, such as a recorder or flageolet, that channels the breath toward the sounding edge of a side opening.
n. An object similar to a fipple in an organ pipe.
n. The block (typically of wood) that forms the floor of the windway in a wind instrument.
the GNU version of the Collaborative International Dictionary of English
n. A stopper, as in a wind instrument of music.
The Century Dictionary and Cyclopedia
n. The under lip.
n. A stopper, as at the mouth of a musical wind-instrument.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a wooden plug forming a flue pipe (as the mouthpiece of a recorder)
Word Usage
"Essentially half of a whistle, stiff side flaps loosely cover the nostrils and mouth; air blown from the nose travels down toward the mouth through a fipple, and the mouth forms the whistle's resonant chamber."