An illusion is a phenomenon that is a misinterpretation of a stimulus. A stimulus is defined as any factor capable of triggering a mechanism in the body. An illusion differs from a hallucination by the fact that in the illusion, a real object is perceived, but it is the interpretation of this perception that is abnormal or inconsistent. In a hallucination however, the stimulus itself does not exist. In both cases, the cause of the error may be due to poisoning, a drug, or an isolated delirium
. In case of persistence without finding a cause, a psychiatric disorder is suspected.