(1):

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

(2):

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

(3):

(n.) 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.