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. A drug or chemical substance that interferes with the physiological action of another, especially by combining with and blocking its receptor.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.