One who contends with another, especially in combat; an adversary; an opponent.
- One who opposes and contends against another; an adversary.
- The principal character in opposition to the protagonist or hero of a narrative or drama.
- Physiology A muscle that counteracts the action of another muscle, the agonist.
- Biochemistry A chemical substance that interferes with the physiological action of another, especially by combining with and blocking its nerve receptor.