'antagonist' definitions:

Definition of 'antagonist'

From: WordNet
noun
Someone who offers opposition [syn: adversary, antagonist, opponent, opposer, resister] [ant: agonist]
noun
A muscle that relaxes while another contracts; "when bending the elbow the triceps are the antagonist"
noun
A drug that neutralizes or counteracts the effects of another drug [ant: synergist]

Synonyms of 'antagonist'

From: Moby Thesaurus