The psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention; SYN. mental disease.
Disordered functioning of the mind. Since normal working cannot easily be defined, the borderline between mild mental illness and normality is a matter of opinion (not to be confused with normative behavior; see norm). It is broadly divided into two categories: neurosis, in which the patient remains in touch with reality; and psychosis, in which perception, thought, and belief are disordered.