A hallucination in the broadest sense 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 and that have qualities of real perception in that they're vivid, substantial, and located in external objective space. These definitions distinguish hallucinations from the related phenomena of dreaming (no consciousness), illusion (distorted or misinterpreted real perception), imagery… (More on Hallucinations)