Opponent

n. rival, adversary; objector; counteracting muscle (Anatomy)

adj. contradicting; opposing

Noun
1. a contestant that you are matched against
(synonym) opposition, opposite
(hypernym) contestant
(derivation) pit, oppose, match, play off
2. someone who offers opposition
(synonym) adversary, antagonist, opposer, resister
(hypernym) person, individual, someone, somebody, mortal, human, soul
(hyponym) dueler, dueller, duelist, duellist
(derivation) oppose
Adjective
1. characterized by active hostility; "opponent (or opposing) armies"
(synonym) opposing
(similar) hostile

n. oppositionist, opposer, objector, refuter, resister; member of the opposition

n. opposer, objector

(n.)
One who opposes; an adversary; an antagonist; a foe.
  

(n.)
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.
  

(a.)
Situated in front; opposite; hence, opposing; adverse; antagonistic.