APA Dictionary of Psychology

an atypical and controversial psychiatric disorder with several forms (e.g., confusional psychosis). Symptoms, which resemble those of a schizoaffective disorder, follow a phasically recurring course and may change rapidly. [as modified in 1957 by German psychiatrist Karl Leonhard (1904–1988) from the original 1924 conceptualization of German neuropsychiatrist Karl Kleist (1879–1960)]