According to psychiatry: "A false perception arising in the absence of an external sensory stimulus" (Lyttle 1986). Hallucinations can be one of the following kinds:
These will usually be the result of mental disorder, although hallucinogenic or "psychotomimetic" drugs can also induce them. Psychotomimetic literally means "mimicking madness", because the effects of these drugs (E.G.
LSD-25,
psilocybin,
mescaline) are similar to some symptoms of
psychotic illnesses.