What is the meaning of mouthpiece?

noun

• A part that goes over or into the mouth of a person.
  • usage: "the mouthpiece of a respirator"
• An acoustic device; the part of a telephone into which a person speaks.
• A spokesperson (as a lawyer.
  • synonyms: mouth
• Especially boxing. ( equipment that protects an athlete's mouth)
  • synonyms: gumshield
• The tube of a pipe or cigarette holder that a smoker holds in the mouth.
• The aperture of a wind instrument into which the player blows directly.
  • synonyms: embouchure