opponent
     adj : characterized by active hostility; "opponent (or opposing)
           armies" [syn: {opposing}]
     n 1: a contestant that you are matched against [syn: {opposition},
           {opposite}]
     2: someone who offers opposition [syn: {adversary}, {antagonist},
         {opposer}, {resister}] [ant: {agonist}]
Source: WordNet® 2.0