a person who is on an opposing side in a game, contest, controversy, or the like; adversary.

being opposite, as in position.

opposing; adverse; antagonistic.

Anatomy . bringing parts together or into opposition, as a muscle.

noun

ally , associate , colleague , helper

noun

adversary , antagonist , anti * , aspirant , assailant , bandit * , bidder , candidate , challenger , competitor , con , contestant , counteragent , dark horse * , disputant , dissentient , enemy , entrant , foe , litigant , match , opposer , opposition , oppugnant , player , rival , oppositionist , resister , competition , contender , corrival , anti , combatant , defendant , dissident , nemesis