Opponent, n.
1. One who opposes; an adversary; an antagonist; a foe. Macaulay.
2. One who opposes in a disputation, argument, or other verbal controversy; specifically, one who attacks some theirs or proposition, in distinction from the respondent, or defendant, who maintains it. How becomingly does Philopolis exercise his office, and seasonably commit the opponent with the respondent, like a long-practiced moderator! Dr. H. More.
Syn. -- Antagonist; opposer; foe. See Adversary.
1 other Webster entry under this spelling
Opponent, a.
Situated in front; opposite; hence, opposing; adverse; antagonistic. Pope.
Shades of meaning
Fernald has no entry of its own for opponent; he lists it under enemy.
Where he writes about it:
An antagonist is one who opposes and is opposed actively and with intensity of effort; an opponent, one in whom the attitude of resistance is the more prominent; a competitor, one who seeks the same object for which another is striving; antagonists in wrestling, competitors in business, opponents in debate may contend with no personal ill will; rivals in love, ambition, etc., rarely avoid inimical feeling. — from enemy
Adversary was formerly much used in the general sense of antagonist or opponent, but is now less common, and largely restricted to the hostile sense; an adversary is ordinarily one who not only opposes another in fact, but does so with hostile spirit, or perhaps out of pure malignity; as, the great Adversary. — from enemy