A sensory perception that has the compelling sense of reality of a true perception but occurs without external stimulation of the relevant sensory organ.
The person may or may not have insight into the fact that he or she is having an hallucination.
Transient hallucinatory experiences may occur without a mental disorder.

Types of hallucinations include:
- auditory
- geometric
- gustatory
- mood-congruent
- mood-incongruent
- olfactory
- somatic
- tactile
- visual

Hallucinations should be distinguished from illusions, in which an actual external stimulus is misperceived or misinterpreted.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th ed. Washington, DC. American Psychiatric Association, 2013. p. 822.