in criminal law an aberration of mind, close to but not actually amounting to insanity, that is caused by mental illness. The defence in Scotland applies only to murder and attempted murder. It is construed a little wider in England where it extends to cover depression or irresistible impulse. In England, the reduced crime is manslaughter and in Scotland culpable homicide (See homicide).
Collins dictionary of law. W. J. Stewart. 2001.