An opponent or enemy.
A muscle that acts in opposition to another.
A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
The main character or force opposing the protagonist in a literary work or drama.
One who antagonizes or stirs.
A side, party (e.g. a team or armed force) seen as adversary, enemy etc. opposite one (or more) other(s), as in a battle or team sports competition.