WordNet sense 1 (the relation between opposed entities):

oppositeness

WordNet sense 2 (a contestant that you are matched against):

opponent, opposite

WordNet sense 3 (an armed adversary (especially a member of an opposing military force)):

enemy, foe, foeman

WordNet sense 4 (the act of hostile groups opposing each other):

confrontation

WordNet sense 5 (the action of opposing something that you disapprove or disagree with):

resistance