fantasy has 5 distinct synonyms spread across 4 of its 4 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.
fiction with a large amount of imagination in it
“she made a lot of money writing romantic fantasies”
something many people believe that is false
“they have the illusion that I am very wealthy”
Synonyms, antonyms and the broader and narrower relations for fantasy come from Princeton University’s WordNet. Full sources and licences.