Antagonist /(ăn*tăg"ō̇*nĭst)/

An·tag·o·nist

Antagonist

n.
  1. One who contends with another, especially in combat; an adversary; an opponent.
    Antagonist of Heaven's Almighty King.
    Our antagonists in these controversies.
  2. A muscle which acts in opposition to another; as a flexor, which bends a part, is the antagonist of an extensor, which extends it. (Anat.)
  3. A medicine which opposes the action of another medicine or of a poison when absorbed into the blood or tissues. (Med.)

Antagonist

a.
  1. Antagonistic; opposing; counteracting; as, antagonist schools of philosophy.