intellect has 4 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.
knowledge and intellectual ability
“he reads to improve his mind”
the capacity for rational thought or inference or discrimination
“we are told that man is endowed with reason and capable of distinguishing good from evil”
a person who uses the mind creatively
Synonyms, antonyms and the broader and narrower relations for intellect come from Princeton University’s WordNet. Full sources and licences.