WordNet records no synonym and no antonym for roman empire. 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.
an empire established by Augustus in 27 BC and divided in AD 395 into the Western Roman Empire and the eastern or Byzantine Empire; at its peak lands in Europe and Africa and Asia were ruled by ancient Rome
Synonyms, antonyms and the broader and narrower relations for roman empire come from Princeton University’s WordNet. Full sources and licences.