A hallucination is a sensory perception of something that does not exist.

Hallucinations can be experienced by any of the five senses, sight, touch, taste, hearing, or smell, but are typically associated with sight and hearing. Hallucinations, which are almost always experienced in a wakened state, should be differentiated from dreaming, which is a normal phenomenon experienced while in a sleeping state.

Hallucinations can be experienced by a single person or, reportedly, by a group of people. Although similar, hallucinations should also be distinguished from mirages, which can usually be explained by physical disturbances in the atmosphere.