antagonist has 4 distinct synonyms spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are agonist, synergist.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a muscle that relaxes while another contracts
“when bending the elbow the triceps are the antagonist”
Synonyms, antonyms and the broader and narrower relations for antagonist come from Princeton University’s WordNet. Full sources and licences.