Wikipedia
Auto-segregation or self-segregation is the separation of a religious or ethnic group from the rest of society in a state by the group itself. This could also mean inability for a normal social interaction and a form of social exclusion.
Through auto-segregation, the members of the separate group can establish their own services, and maintain their own traditions and customs. For example, some of the world's uncontacted peoples have preferred not to interact with the rest of the globally integrated human population. By remaining in a reserve and in isolation, they can preserve their cultures intact as long as they choose and the surrounding states protect them.
On the other hand, some groups self-segregate to avoid integrating with other groups, either to protect their culture or some may self-segregate from others that they consider intellectually and/or socially inferior. Nationalists (non-racists) or racists, ultranationalists, different Hindu castes and other 'supremacist' or ethno-centric groups commonly segregate themselves from other communities through various practices like endogamy. The apartheid system in South Africa was an example of this trend.