opposing

adjective

1. Acting against or in opposition: adversarial, adverse, antagonistic, antipathetic, opposed, oppositional. See support/oppose.
2. Diametrically opposed: antipodal, antipodean, antithetical, antonymic, antonymous, contradictory, contrary, converse², counter, diametric, diametrical, opposite, polar, reverse. See support/oppose.

The adjective has 3 meanings:

Meaning #1: characterized by active hostility
  Synonym: opponent

Meaning #2: acting in opposition to

Meaning #3: in opposition to (a policy or attitude etc.)
  Synonym: opposed