hallucination 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.
illusory perception; a common symptom of severe mental disorder
a mistaken or unfounded opinion or idea
“he has delusions of competence”
Synonyms, antonyms and the broader and narrower relations for hallucination come from Princeton University’s WordNet. Full sources and licences.
an object perceived during a hallucinatory episode
“he refused to believe that the angel was a hallucination”