Table 1.

Definitions of hallucination, illusion, and distortion [after (20)].

Hallucination	A percept, experienced by a waking individual, in the absence of an appropriate stimulus from the outside world (e.g., seeing a cat that is not there, hearing a voice in the absence of sound waves)
Illusion	A percept, experienced by a waking individual, which is based on an appropriate stimulus from the outside world, and which is either misperceived or misinterpreted (e.g., taking a moving curtain for an intruder, hearing music in the monotonous drone of a computer fan)
Distortion	A percept, experienced by a waking individual, which is based on an appropriate stimulus from the outside world, of which, however, a highly specific aspect is altered in a consistent manner (e.g., seeing all straight lines as wavy, feeling one's head grow to an unnaturally large size)