psychosis (sīkō´sĬs), in psychiatry, a broad category of mental disorder encompassing the most serious emotional disturbances, often rendering the individual incapable of staying in contact with reality. Until recently, the term was used in contrast with neurosis, which denoted the "mild" mental disorders which did not interfere significantly with the ability to function normally, or severely impair the individual's conception of reality. In 1980, the American Psychiatric Association made sweeping changes in its classificatory system for psychological disorders, and the opposition between neurosis and psychosis became obsolete. The former classification included functional psychoses including schizophrenia, paranoia, bipolar disorder, and involutional psychotic reactions, where no brain change was detectable with available tools. Today, there are separate categories for schizophrenic disorders, mood disorders (which include bipolar disorder and major depression), and other serious mental disturbances such as delusional disorder. Symptoms of these disorders may include hallucinations and delusions; severe deviations of mood (depression and mania); lack of, or inappropriateness of, emotional response; and severe impairment of judgment. Another type of psychosis involves brief episodes, characterized by an acute onset lasting no longer than a month, usually resulting from situational circumstances such as an earthquake or flood. Nonspecified psychotic disorders include psychotic symptoms, e.g., delusions, hallucinations, or disorganized behavior, that cannot be classified in any other disorder. Drug therapy and electroconvulsive therapy have been successful in the treatment of many patients with serious psychological disorders. Organic psychoses, so called because of the structural deterioration of the brain, include senile dementia and Alzheimer's disease. Occurring in middle to old age, these disorders involve progressive, nonreversible brain damage. Organic brain damage may also result from toxic reactions to such substances as alcohol, PCP, amphetamines, and crack cocaine. In criminal law, the term insanity can be applied to most forms of psychoses, although defenses based on insanity have been relatively rare.