A hallucination is a sensation or sensory perception that a person experiences in the absence of a relevant external stimulus. That is, a person experiences something that doesn’t really exist (except in their mind). A hallucination can occur in any sensory modality — visual, auditory, olfactory, gustatory, tactile, etc.
Auditory hallucinations (e.g. hearing voices or some other sound) are most common type of hallucination in schizophrenia. Visual hallucinations are also relatively common. Research suggests that auditory hallucinations occur when people misinterpret their own inner self-talk as coming from an outside source.
Hallucinations can often be meaningful to the person experiencing them. Many times, the voices are those of someone they know. Most commonly, the voices are critical, vulgar, or abusive. Hallucinations also tend to be worse when the person is alone.
The patient complains of an overwhelming chemical smell in and around his room; no one else smells it, although he insists it’s still there and says he feels faint.
Grinnell, R. (2011). Hallucination. Psych Central. Retrieved on October 7, 2015, from http://psychcentral.com/encyclopedia/2008/hallucination/