A hallucination is a perception of something that is not really there. A hallucination can involve any of the senses: hearing, sight, smell, taste, or touch.

The most common hallucinations are:

• Auditory hallucinations, or hearing things. For example, the person may hear a voice calling his or her name or hear many voices talking, sometimes saying things that are frightening. People with schizophrenia may hear voices telling them to do things (command hallucinations), such as harm themselves or someone else. Auditory hallucinations are the most common type.
• Visual hallucinations, or seeing things. For example, a person with schizophrenia may see another person, an animal, or an object that other people do not see. Sometimes visual hallucinations are pleasant, and sometimes they are frightening.

Other types of hallucinations include tasting something (gustatory hallucination), smelling something (olfactory hallucination), and feeling something that is not there (tactile hallucination).

Some people have hallucinations that are not considered symptoms of mental illness. They may occur when a person is about to fall asleep (hypnagogic hallucinations) or when a person is just waking up (hypnopompic hallucinations).