(1):
(n.) A medicine which opposes the action of another medicine or of a poison when absorbed into the blood or tissues.
(2):
(n.) A muscle which acts in opposition to another; as a flexor, which bends a part, is the antagonist of an extensor, which extends it.
(3):
(n.) One who contends with another, especially in combat; an adversary; an opponent.
(4):
(a.) Antagonistic; opposing; counteracting; as, antagonist schools of philosophy.