All 3 Uses

antagonist
in
A Connecticut Yankee in King Arthur's Court
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The best swordsman in the world doesn't need to fear the second best swordsman in the world; no, the person for him to be afraid of is some ignorant antagonist who has never had a sword in his hand before; he doesn't do the thing he ought to do, and so the expert isn't prepared for him; he does the thing he ought not to do; and often it catches the expert out and ends him on the spot.†
Chpt 34

antagonist = someone who offers opposition or is hostile
I tried to imagine what would happen when the master should discover that it was I who had been fighting him; and what would happen if they jailed us together in the general apartment for brawlers and petty law-breakers, as was the custom; and what might— Just then my antagonist turned his face around in my direction, the freckled light from the watchman's tin lantern fell on it, and, by George, he was the wrong man!†
Chpt 36
It was known that Merlin had been busy whole days and nights together, imbuing Sir Sagramor's arms and armor with supernal powers of offense and defense, and that he had procured for him from the spirits of the air a fleecy veil which would render the wearer invisible to his antagonist while still visible to other men.†
Chpt 39 *

Definitions:

(1) (antagonist as in: the antagonist in the film)
someone who offers opposition or is hostile
(2) (meaning too rare to warrant focus)
Specialized Definitions of antagonist:
- literature or storytelling: the most important character who opposes the main character or protagonist
- physiology: a muscle that relaxes when another muscle contracts
- chemistry and biology: a substance that interferes with the effect of another substance