A hallucination is a perception of something that does not correspond to external stimuli, and may involve any of the five senses (or any combination of them). It can be the result of a number of causes including drugs, sleep deprivation, psychosis or neurological dysfunction.
A rarely expressed but persistent alternate explanation of hallucinations, espoused only by non-materialists, is that "crazy" people can sometimes perceive non-physical phenomena such as angels, visions or the voices of departed spirits or demons.
However one may interpret the causality and reality, i.e. relevance of hallucinations, it seems to be clear that their occurrence strongly correlates with two extremes of sensory activation: either sensory flooding[?] (sensory overload[?]) or sensory deprivation[?]. Both, of course, may be caused by external (as rave party or solitude[?]) or internal factors (as drugs, brain injury[?], illness, or sleep deprivation).
The first usage of the word 'hallucination' in the English language is recorded as by the English physician Sir Thomas Browne in 1642.