One who opposes another physically (in a fight, sport, game, or competition).
The participant who opens an academic debate by putting forward objections to a theological or philosophical thesis.
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.
One who opposes another in words (in a dispute, argument or controversy).
One who is opposed or objects (to a policy, course of action or set of ideas).
A side, party (e.g. a team or armed force) seen as adversary, enemy etc. opposite one (or more) other(s), as in a battle or team sports competition.