gravimeter has 2 distinct synonyms spread across 2 of its 2 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 measuring instrument for determining the specific gravity of a liquid or solid
a measuring instrument for measuring variations in the gravitational field of the earth
Synonyms, antonyms and the broader and narrower relations for gravimeter come from Princeton University’s WordNet. Full sources and licences.