(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.