Schizophrenia: a long-term mental disorder of a type involving a breakdown in the relation between thought, emotion, and behavior, leading to faulty perception, inappropriate actions and feelings, withdrawal from reality and personal relationships into fantasy and delusion, and a sense of mental fragmentation.
The word 'schizophrenia' literally means "split mind" but is not characterized by the presence of multiple personalities. Common symptoms of schizophrenia include delusions, hallucinations, disorganized thinking and speech, and a reduced ability to function normally (ie. a loss of motivation or interest, withdrawal from relationships).
Delusion: a firmly held belief that is not based in reality; there may even be evidence to the contrary
Hallucination: sensing something that is not there. Audible hallucinations (that is, hearing voices) are commonly experienced by people with schizophrenia.