C2[C or U]an idea or belief that is not true: He had no illusions about his talents as a singer.I'm under no illusions (= I understand the truth) about the man I married.My boss is labouring under the illusion that(= wrongly believes that) the project will be completed on time.Unreal things and unrealityC2[C]something that is not really what it seems to be: A large mirror in a room can create the illusion of space.The impression of calm in the office is just an illusion.Hiding and disguising