This word lives with the belief that it is important and good to include people and ideas from many countries, races, and religions. A multiculturalist is, most often, a person who holds that belief. You can also use it as an adjective: a multiculturalist view, a multiculturalist policy — the same camp, used to describe the view or the people in it.

Though actually, the ending is doing a job students mix up. -ism is the belief (multiculturalism). -ist is usually the person, or the adjective for that camp. If you say “she is a multiculturalist,” you are naming a person, not the idea itself. Some loose writing treats multiculturalist as if it were the belief; a careful speaker will not copy that.

Like its cousin, this word lives in news and argument. Some people wear it proudly. Some people spit it, as if a multiculturalist were naive or “political” in a bad way. The word itself does not tell you which feeling the speaker has — the rest of the sentence does. It is a little formal. Everyday English might just say “people who believe in multiculturalism.”