An illusion is a sensory distortion that occurs when the brain misinterprets or adds information to what the eyes see. Illusions can affect any of the senses, but visual illusions are the most well-understood.These are visual misinterpretations that can occur when the brain fills in gaps in incomplete information. For example, a pencil in a glass of water may appear bent or broken.

Illusions are special perceptual experiences in which information arising from “real” external stimuli leads to an incorrect perception, or false impression, of the object or event from which the stimulation comes. Illusions are thought to occur when stimuli from a perceived object are combined with mental images to produce a false perception. Illusions are associated with inattention when external sensory stimuli are meagre or when attention is impaired due to delirium.