1. a false sensory percept. Illusions of the senses, such as visual illusions, result from the misinterpretation of sensory stimuli. For example, parallel railroad tracks appear to meet in the distance (see alley problem; linear perspective). Other examples of visual illusions are apparent movement, contrast illusions, distortion illusions (such as the Hering illusion, Müller-Lyer illusion, Poggendorf illusion, Ponzo illusion, and Zöllner illusion), and the Panum phenomenon. Illusions involving other senses include Aristotle’s illusion. All of these illusions occur under ordinary conditions. The distorted percepts that may accompany abnormal conditions, such as delirium and schizophrenia, or that occur in those taking mind-altering drugs are more properly called hallucinations.

2. a distortion in memory (see memory illusion), such as déjà vu.

3. more generally, any false belief or mistaken idea. —illusory adj.