realisation has 5 distinct synonyms spread across 6 of its 6 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.
a musical composition that has been completed or enriched by someone other than the composer
coming to understand something clearly and distinctly
“a growing realization of the risk involved”
a sale in order to obtain money (as a sale of stock or a sale of the estate of a bankrupt person) or the money so obtained
the completion or enrichment of a piece of music left sparsely notated by a composer
making real or giving the appearance of reality
something that is made real or concrete
“the victory was the realization of a whole year's work”
Synonyms, antonyms and the broader and narrower relations for realisation come from Princeton University’s WordNet. Full sources and licences.