A hallucination is a fact, not an error. What is erroneous is a judgment based upon it

Don Quijote de la Mancha, Pablo Picasso, 1937.

The word 'hallucinatory' has its roots in the Latin hallucinere or allucinere, meaning 'to wander in mind'. Altenatively in the Greek aλύσκειν, (haluskein), meaning flee from, shun, avoid, escape.

A hallucination is the brain's reception of a false sensory input. This essentially means that the person having a hallucination is experiencing an event through one of their senses that is not occurring in the real world. This can be through any of the senses, with tactile then auditory hallucinations being the most common. When auditory hallucinations are examined, the most common are hearing one's own thoughts as if they were being spoken aloud, followed by hearing one's name being called by a voice when alone.
People may experience hallucinations as part of their normal developmental stages, especially during the preschool years, in the 2-5 year old range. Common causes of hallucinations in people without a psychiatric diagnosis are exhaustion, sleep deprivation, social isolation and rejection, severe reactive depression, amputation of a limb (phantom limb syndrome), a reaction to medication, a reaction to hallucinogens such as LSD, a reaction to other drugs such as heroin and cocaine.
Physical (organic) causes of hallucinations include delirium, tumors leading to increased intercranial pressure, temporal lobe lesions, seizures, head injuries, and irritation of sensory pathways.
In persons with a psychiatric diagnosis, it is the psychotic disorders that are generally associated with hallucinations. These include all of the schizophrenic diagnoses, and affective disorders (such as bipolar disorder with psychotic features).