An Opponent

1. One who opposes another; one who works or takes a position against someone or something; one who attempts to stop the progressof someone or something.
   1. One who opposes another physically (in a fight, sport, game, or competition).
   2. One who opposes another in words (in a dispute, argument or controversy).
   3. One who is opposed or objects (to a policy, course of action or set of ideas).
   4. (historical) The participant who opens an academic debate by putting forward objections to a theological or philosophicalthesis.

Synonyms

• adversary
• antagonist

Antonyms

• (one who opposes): supporter, proponent, advocate
• (person who opens an academic debate):respondent