a sensory perception in the absence of an external stimulus. Hallucinations are often differentiated from sensory illusions which are distortions or misinterpretations of actual sensory experiences. Hallucinations can involve any sensory modality (visual, auditory, tactile, olfactory, or gustatory). Simple (unformed) hallucinations are sensory perceptions that are typically vague and without meaning (e.g., whistling sounds, flashing lights, geometric patterns).

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.