Assimilation is when a people mixes genetically and culturally into another group, lossing theirs original identity.
Assimilation existed, for example, when the French hunters married Native women in Quebec and theirs children spoke only French and followed only French culture.
Assimilation does not happens when a group is kept apart from the mainstream. The typical case are the appartheid in South Africa, and also most Black people in the United States. They were kept appart from the mainstream and not allowed to fuse into it genetically and culturally.
In Brazil, the same Black people are integrated to the society because they share both genetics and culture with the rest of the population. Therefore, they are assimilated.
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