belly has 5 distinct synonyms spread across 3 of its 6 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.
the region of the body of a vertebrate between the thorax and the pelvis
a protruding abdomen
Synonyms, antonyms and the broader and narrower relations for belly come from Princeton University’s WordNet. Full sources and licences.
a part that bulges deeply
“the belly of a sail”
the hollow inside of something
“in the belly of the ship”
the underpart of the body of certain vertebrates such as snakes or fish