multiculturalism has no recorded synonym. 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. Its one recorded opposite is nationalism.
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 doctrine that several different cultures (rather than one national culture) can coexist peacefully and equitably in a single country
Synonyms, antonyms and the broader and narrower relations for multiculturalism come from Princeton University’s WordNet. Full sources and licences.