opponent

noun

1 he beat his opponent by a landslide margin:

rival, adversary, opposer, the opposition, fellow contestant, (fellow) competitor, other competitor/contestant/player/candidate, enemy, foe, antagonist, combatant, contender, challenger, critic, dissenter, disputant, objector.

▷antonyms ally, partner, colleague.

2 an opponent of the economic reforms:

opposer, objector, dissident, dissenter, rejectionist.

▷antonyms supporter.