[Disease, syndrome, sequence].

A brief review is given of the historical and modern use of the terms "disease," "syndrome," and "symptom complex." In the old use, notably the Greek school of empirics, phenotype and disease were thought to be almost synonymous; then Thomas Sydenham added etiology and pathogenesis as defining criteria. Today the terms "disease," "syndrome," and "symptom complex" are used to signal varying degrees of medical knowledge and thus direct medical activity. In accordance with modern trends, a "disease" is understood as an etiologically und pathogenetically defined entity, a "syndrome" as an etiologically defined entity of unknown pathogenesis, and a "sequence" as an etiologically heterogeneous but pathogenetically defined disorder. None of these terms should be used for "symptom complexes," i.e., for causally undefined or heterogeneous phenotypes.