A hallucination, in the broadest sense,'s 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're vivid, substantial, and located in external objective space. The latter definition distinguishes hallucinations from the related phenomena of dreaming, which doesn't involve consciousness; illusion, which involves distorted or mi… (More on Hallucination)