- One who opposes and contends against another; an adversary. (noun)
- The principal character in opposition to the protagonist or hero of a narrative or drama. (noun)
- Physiology A muscle that counteracts the action of another muscle, the agonist. (noun)
- Biochemistry A chemical substance that interferes with the physiological action of another, especially by combining with and blocking its nerve receptor. (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.