Antagonist
ANTAG'ONIST, noun [Gr. against, and a champion. See Act and Agony.]
1. One who contends with another in combat; used primarily in the Grecian games. An adversary.
2. An opponent in controversy.
3. In anatomy, a muscle which acts in opposition to another; as a flexor, which bends a part, is the antagonist of an extensor, which extends it.
ANTAG'ONIST, adjective Counteracting; opposing; combating; as, an antagonist muscle.