republic has 2 distinct synonyms spread across 1 of its 2 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 political system in which the supreme power lies in a body of citizens who can elect people to represent them
a form of government whose head of state is not a monarch
Synonyms, antonyms and the broader and narrower relations for republic come from Princeton University’s WordNet. Full sources and licences.
“the head of state in a republic is usually a president”