Definitions of 'Hallucination' include:
Hallucination: A perception experienced in the
absence of an external stimulus to the sense organs, with a similar
quality to a true perception. May be experienced by tired normal
people, and not infrequently during the transition from sleeping to
waking. It's only a psychotic symptom if the subject loses touch
with external reference: e.g. a vision experienced during an LSD
trip which the subject knows is due to the effect of the drug is not
psychotic. Hallucination has to be distinguished from an illusion (a
misperception of an external stimulus).
Hallucination: A profound distortion in a
person's perception of reality, typically accompanied by a powerful
sense of reality. An hallucination may be a sensory experience in
which a person can see, hear, smell, taste, or feel something that
is not there.
link to: DELIRIUM