an·tag·o·nist

[an-tag-uh-nist]
noun
1.
a person who is opposed to, struggles against, or competes with another; opponent; adversary.
2.
the adversary of the hero or protagonist of a drama or other literary work: Iago is the antagonist of Othello.
3.
Physiology. a muscle that acts in opposition to another. Compare agonist ( def 3 ).
4.
Dentistry. a tooth in one jaw that articulates during mastication or occlusion with a tooth in the opposing jaw.
5.
Pharmacology. a drug that counteracts the effects of another drug.
Origin:
1590–1600; < Late Latin antagōnista < Greek antagōnistḗs. See antagonize, -ist

Can be confused: antagonist, proponent, protagonist.

Synonyms
1. contestant, enemy, foe. See adversary.

Antonyms
1. ally, friend.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2013.
Cite This Source Link To antagonist
00:10
Antagonist is always a great word to know.
So is reaction. Does it mean:
action in response to a stimulus of a system, a nerve or a muscle
an innate periodicity in an organism's physiological processes like sleep and wake cycles
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Collins
World English Dictionary
antagonist (ænˈtæɡənɪst) [Click for IPA pronunciation guide]
 
—n
1.  an opponent or adversary, as in a contest, drama, sporting event, etc
2.  Compare agonist any muscle that opposes the action of another
3.  Compare synergist a drug that counteracts the effects of another drug

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

antagonist
c.1600, from L. antagonista, from Gk. antagonistes "competitor, opponent, rival," from antagonizesthai "to struggle against," from anti- "against" + agonizesthai "to contend for a prize," from agon "contest" (see agony). Originally in battle or sport, extended 1620s to any
sphere of human activity. Related: Antagonistic (1630s).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

antagonist an·tag·o·nist (ān-tāg'ə-nĭst)
n.
Something, such as a muscle, disease, or physiological process, that neutralizes or impedes the action or effect of another.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
antagonist   (ān-tāg'ə-nĭst)  Pronunciation Key 

  1. A muscle that opposes the action of another muscle, as by relaxing while the other one contracts, thereby producing smooth, coordinated movement.

  2. A chemical substance, such as a drug, that interferes with the physiological action of another substance, especially by combining with and blocking its nerve receptor. Compare agonist.


The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
As it turned out — and always does in the movies — the antagonist
  miscalculated.
Indeed, they seemed to have given up any idea of gaining any advantage of their
  antagonist in the open field.
But this antagonist is real, and it affects the brains of tens of thousands of
  people every year.
It's not love that Conroy feels for his old antagonist but a sense of triumph.
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