A psychological disorder for which no organic cause can be found.

Disorders traditionally classified as neuroses (including a variety of anxiety and mood disorders as well as psychosomatic illnesses) are generally regarded as functional disorders. While conditions classified as psychotic are usually believed to have biological origins, neurotic conditions are generally believed to be caused by developmental, psychosocial, or personality factors. Psychotic disorders not associated with damage to brain tissue from a head injury, infection, or similar causes are also considered functional disorders.