Source : Webster's Revised Unabridged Dictionary (1913)

Mouthpiece \Mouth"piece`\, n.
   1. The part of a musical or other instrument to which the
      mouth is applied in using it; as, the mouthpiece of a
      bugle, or of a tobacco pipe.

   2. An appendage to an inlet or outlet opening of a pipe or
      vessel, to direct or facilitate the inflow or outflow of a
      fluid.

   3. One who delivers the opinion of others or of another; a
      spokesman; as, the mouthpiece of his party.

Source : WordNet®

mouthpiece
     n 1: a part that goes over or into the mouth of a person; "the
          mouthpiece of a respirator"
     2: an acoustic device; the part of a telephone into which a
        person speaks
     3: a spokesperson (as a lawyer) [syn: {mouth}]
     4: (especially boxing) equipment that protects an athlete's
        mouth [syn: {gumshield}]
     5: the tube of a pipe or cigarette holder that a smoker holds
        in the mouth
     6: the aperture of a wind instrument into which the player
        blows directly [syn: {embouchure}]