1. illusion, hallucination, delusion refer to false perceptions or ideas. An illusion is a false mental image produced by misinterpretation of things that actually exist: A mirage is an illusion produced by reflection of light against the sky. A hallucination is a perception of a thing or quality that has no physical counterpart: Under the influence of LSD, Terry had hallucinations that the living-room floor was rippling. A delusion is a persistent false belief: A paranoiac has delusions of persecution.

Did you mean illusion?