psychosis

psychosis, any of several major mental illnesses that can cause delusions, hallucinations, serious defects in judgment and other cognitive processes, and the inability to evaluate reality objectively. A brief treatment of psychosis follows. For full treatment, see mental disorder. The term psychosis is derived from the Greek psyche, meaning “soul,” “mind,” or “breath.” The ancient Greeks believed that the breath was the animating force of life and that when the breath left the body, as...