The term "racist" used to mean someone who believed that one's race, or ethnic background was superior to another's. It was a form of belief in racial supremacy.

It didn't mean that someone used race as a form of prejudice. But for decades this hasn't been the case, and it is almost exclusively used to describe someone who uses prejudice or discriminates others on the basis of race or ethnicity.

And now we no longer have a colloquial or formal single word for someone who holds racial supremacist views, because the two ideas have commingled.

Sorry, what's the difference? If you think A is superior to B, that also means that B is inferior to A? And without empiricism, that is a prejudice? And when you can decide between things, that is also discriminating?

It is hardly surprising that people don't bother differentiating between those that think they are inherently superior by dint of their race versus those that think others are innately inferior by dint of their race. The one implies the other. And those who would impose such views when met with counterexamples, with condescension, mockery, shunning, exclusion, and violence, are never so far behind.

Maybe I'm misremembering, but people used to use "racist" (and "sexist") as adjectives, not nouns. You'd say "Joe is racist", not "Joe is a racist". The latter seems more common now, particularly in American English, but just sounds wrong to me.

For the vast majority of the situations in a English-speaking society, the term for that type of person is a "white supremacist". For example, a person who does Nazi salutes, or wants to update refugee policy so that the majority of immigrants are White South Africans, is a white supremacist.