Transient is most often used to modify nouns like nature, threat, source and cause, which suggests that the word often shows up in formal contexts, like analysis of finance or global terrorism. But it can also be used for anything that moves quickly from one thing to another, like a transient feeling or facial expression. Transient is also a noun meaning "a person who moves from place to place; a homeless person." The word comes from Latin transire, "to pass over," so you can think of it as describing things that are quickly passed over.