Illusion , a misrepresentation of a “real” sensory stimulus —that is, an interpretation that contradicts objective “reality” as defined by general agreement. For example, a child who perceives tree branches at night as if they are goblins may be said to be having an illusion . An illusion is distinguished from a hallucination , an experience that seems to originate without an external source of stimulation. Neither experience is necessarily a sign of psychiatric disturbance, and both are regularly and consistently reported by virtually everyone. The nature of illusions 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. Some of these false impressions may arise from factors beyond an individual’s control (such as the characteristic behaviour of light waves that makes a pencil in a glass of water seem bent), from i