A hallucination is a perception in the absence of external stimulus that has qualities of real perception. Hallucinations are vivid, substantial, and located in external objective space. They are distinguished from the related phenomena of dreaming, which does not involve wakefulness; illusion, which involves distorted or misinterpreted real perception; imagery, which does not mimic real perception and is under voluntary control; and pseudohallucination, which does not mimic real perception, but is not under voluntary control.
Auditory hallucinations of the malicious type are frequently heard, for example people talking about the patient behind his/her back. Like auditory hallucinations, the source of the visual counterpart can also be behind the patient's back. Their visual counterpart is the feeling of being looked or stared at, usually with malicious intent. Frequently, auditory hallucinations and their visual counterpart are experienced by the patient together
Hallucinations can be associated with drug use, sleep deprivation, psychosis and neurological disorders.