Webster's English Dictionary

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antagonist

Cross references:
  1. opponent              
 
an.tag.o.nist \an-'tag-*-n*st\ n 1: one that opposes another esp. in combat 
   : ADVERSARY  2: an agent of physiological antagonism : as  2a: a muscle 
   that contracts with and limits the action of an agonist with which it is 
   paired 2b: a drug that opposes thae action of another