WordNet records no synonym and no antonym for roman republic. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
the ancient Roman state from 509 BC until Augustus assumed power in 27 BC; was governed by an elected Senate but dissatisfaction with the Senate led to civil wars that culminated in a brief dictatorship by Julius Caesar
Synonyms, antonyms and the broader and narrower relations for roman republic come from Princeton University’s WordNet. Full sources and licences.