constituent has 11 distinct synonyms spread across 5 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.
an artifact that is one of the individual parts of which a composite entity is made up; especially a part that can be separated from or attached to a system
“spare components for cars”
a member of a constituency; a citizen who is represented in a government by officials for whom he or she votes
“needs continued support by constituents to be re-elected”
(grammar) a word or phrase or clause forming part of a larger grammatical construction
an abstract part of something
“jealousy was a component of his character”
constitutional in the structure of something (especially your physical makeup)
Synonyms, antonyms and the broader and narrower relations for constituent come from Princeton University’s WordNet. Full sources and licences.