Main Entry: an·tag·o·nist

Pronunciation: \-nist\

Function: noun

Date: 1575

1 : one that contends with or opposes another : adversary, opponent
2 : an agent of physiological antagonism: as a : a muscle that contracts with and limits the action of an agonist with which it is paired —called also antagonistic muscle b : a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate); especially : one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor — compare agonist 2b