imagination has 7 distinct synonyms spread across 3 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.
the formation of a mental image of something that is not perceived as real and is not present to the senses
“popular imagination created a world of demons”
the ability to form mental images of things or events
“he could still hear her in his imagination”
the ability to deal resourcefully with unusual problems
“a man of resource”
Synonyms, antonyms and the broader and narrower relations for imagination come from Princeton University’s WordNet. Full sources and licences.