noun One who opposes another; one who works or takes a position against someone or something; one who attempts to stop the progress of someone or something.
The person who ran against her in the last election proved to be a formidable opponent.
noun One who opposes another physically (in a fight, sport, game, or competition).
noun One who opposes another in words (in a dispute, argument or controversy).
noun One who is opposed or objects (to a policy, course of action or set of ideas).
She was a dedicated opponent of the death penalty.
noun The participant who opens an academic debate by putting forward objections to a theological or philosophical thesis.