- One involved in a struggle or competition. (noun)
- Physiology A contracting muscle that is resisted or counteracted by another muscle, the antagonist. (noun)
- Biochemistry A drug or other chemical that can combine with a receptor on a cell to produce a physiologic reaction typical of a naturally occurring substance. (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.