A hallucination, in the broadest sense of the word, is a
perception in the absence of a stimulus. In a stricter sense,
hallucinations are defined as perceptions in a conscious and
awake state in the absence of external stimuli which have
qualities of real perception, in that they are vivid,
substantial, and located in external objective space. The latter
definition distinguishes hallucinations 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.
Hallucinations also differ from "delusional perceptions", in
which a correctly sensed and interpreted stimulus (i.e. a real
perception) is given some additional (and typically bizarre)
significance. A mild form of hallucination is known as a
disturbance, and can occur in any of the senses above. These m