A body of rock forming a discrete and definable unit. Such units are determined on the basis of their lithology (lithostratigraphic units), or their fossil content (biostratigraphic units), or their time span (chronostratigraphic units). It is unlikely that any rock succession will form a unit that accords with all three categories of classification. All stratigraphic units are defined by a type section. Geologic-time units are abstract concepts, not actual rock sequences, so do not class as stratigraphic units.