assimilate has 4 distinct synonyms spread across 2 of its 5 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. Its one recorded opposite is dissimilate.
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.
become similar to one's environment
“Immigrants often want to assimilate quickly”
make similar
“This country assimilates immigrants very quickly”
become similar in sound
“The nasal assimilates to the following consonant”
Synonyms, antonyms and the broader and narrower relations for assimilate come from Princeton University’s WordNet. Full sources and licences.