An illusion is a distortion of a sensory perception. Each of the human senses can be deceived by illusions, but visual illusions are the most well known. Some illusions are subjective; different people may experience an illusion differently, or not at all.

• Optical illusions, such the use of false perspective, exploit assumptions made by the human visual system.
• Mirages are optical distortions through the atmosphere that may be photographed. While the perceived reality (such as water in the desert) is illusory, the visual image (of a reflective surface) is real.
• Auditory illusions, such as the Shepard Tone, exploit our hearing.
• Touch illusions exploit our sense of touch.
• Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred. See magic (illusion).

In psychiatry the term illusion refers to a specific form of sensory distortion. Unlike an hallucination, which is a sensory experience in the absence of a stimulus, an illusion describes a distortion of a perception so it is understood and interpreted differently. For example, hearing voices regardless of the environment would be an hallucination, whereas hearing voices which arise only from the sound of running water (or other auditory source) would be an illusion.

See also

• delusion
• hallucination
• cognitive bias
• simulated reality for a more radical approach to the possibility of illusion
• Hermann von Helmholtz

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