assimilation has 2 distinct synonyms spread across 3 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.
the state of being assimilated; people of different backgrounds come to see themselves as part of a larger national family
the social process of absorbing one cultural group into harmony with another
the process of absorbing nutrients into the body after digestion
a linguistic process by which a sound becomes similar to an adjacent sound
the process of assimilating new ideas into an existing cognitive structure
in the theories of Jean Piaget: the application of a general schema to a particular instance
Synonyms, antonyms and the broader and narrower relations for assimilation come from Princeton University’s WordNet. Full sources and licences.