Real Dictionary

white supremacy in American English
the belief, theory, or doctrine that the White race is superior to all other races, esp. the Black race, and should therefore retain control in all relationships
the social, economic, and political repression and exploitation of nonwhite peoples, esp. blacks, by white people, based on notions of racial superiority

White supremacy in British English
noun: the theory or belief that White people are innately superior to people of other races