n.
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