Definition of "Mouthpiece" :

noun: the aperture of a wind instrument into which the player blows directly

noun: the tube of a pipe or cigarette holder that a smoker holds in the mouth

noun: an acoustic device; the part of a telephone into which a person speaks

noun: (especially boxing) equipment that protects an athlete's mouth

noun: a part that goes over or into the mouth of a person

"The mouthpiece of a respirator."

noun: a spokesperson (as a lawyer)