empire has 2 distinct synonyms spread across 2 of its 5 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.
Synonyms, antonyms and the broader and narrower relations for empire come from Princeton University’s WordNet. Full sources and licences.
a group of countries under a single authority
“the British created a great empire”
a monarchy with an emperor as head of state
a group of diverse companies under common ownership and run as a single organization
an eating apple that somewhat resembles a McIntosh; used as both an eating and a cooking apple