Hallucinations

Hallucinations are false or distorted sensory experiences that appear to be real perceptions. When you hallucinate, you are hearing, seeing, smelling or feeling things that do not actually exist. The most common hallucinations are auditory – people will hear voices talking about them or to them. Although these experiences are not real, they seem as vivid and real as any other experience.

Hallucinations are usually diagnosed alongside such psychotic disorders as: dementia, bipolar disorder (manic depression), schizophrenia, substance abuse, as well as with some personality disorders. Schizophrenia involves delusions, hallucinations, disturbances in thinking or disorganized behaviour at some phase of the illness.
 
Hallucinations are different from delusions and paranoia.  If you or someone you know is experiencing delusions, you should seek medical advice.