A subfield of psychiatry that emphasizes the somatic substructure on which mental operations and emotions are based, and the functional or organic disturbances of the central nervous system that give rise to, contribute to, or are associated with mental and emotional disorders. (From Campbell's Psychiatric Dictionary, 8th ed.)

Psychiatric illness or diseases manifested by breakdowns in the adaptational process expressed primarily as abnormalities of thought, feeling, and behavior producing either distress or impairment of function.