adjunct has 6 distinct synonyms spread across 2 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.
something added to another thing but not an essential part of it
a person who is an assistant or subordinate to another
a construction that can be used to extend the meaning of a word or phrase but is not one of the main constituents of a sentence
of or relating to a person who is subordinate to another
Synonyms, antonyms and the broader and narrower relations for adjunct come from Princeton University’s WordNet. Full sources and licences.