region has 4 distinct synonyms spread across 4 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.
a part of an animal that has a special function or is supplied by a given artery or nerve
“in the abdominal region”
a large indefinite location on the surface of the Earth
“penguins inhabit the polar regions”
the approximate amount of something (usually used prepositionally as in `in the region of')
“it was going to take in the region of two or three months to finish the job”
a knowledge domain that you are interested in or are communicating about
“it was a limited realm of discourse”
Synonyms, antonyms and the broader and narrower relations for region come from Princeton University’s WordNet. Full sources and licences.