instrumentality has 1 distinct synonym spread across 1 of its 3 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 subsidiary organ of government created for a special purpose
“are the judicial instrumentalities of local governments adequate?”
the quality of being instrumental for some purpose
an artifact (or system of artifacts) that is instrumental in accomplishing some end
Synonyms, antonyms and the broader and narrower relations for instrumentality come from Princeton University’s WordNet. Full sources and licences.