1. Of the same kind but situated opposite, exerting energy in the opposite direction, or pursuing an opposite policy; one that is opposite in kind to
2. Opposing or hostile to in opinion, sympathy, or practice; opposing in effect or activity
3. Serving to prevent, cure, or alleviate
4. Combating or defending against

Not in favor of (an action or proposal etc.).

One that is opposed.

Opposed to; against