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

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.