Opponent

Webster's Dictionary of the English Language

·noun One who opposes; an adversary; an antagonist; a foe.

II. Opponent ·adj Situated in front; opposite; hence, opposing; adverse; antagonistic.

III. Opponent ·noun One who opposes in a disputation, argument, or other verbal controversy; specifically, one who attacks some theirs or proposition, in distinction from the respondent, or defendant, who maintains it.