A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are generally divided into individual formations. Groups may sometimes be divided into "subgroups" and are themselves sometimes grouped into "supergroups".
Some well known groups of northwestern Europe have in the past also been used as units for chronostratigraphy and geochronology. These are the Rotliegend, Zechstein (both of Permian age), Buntsandstein, Muschelkalk, Keuper (Triassic in age), Lias, Dogger and Malm (Jurassic in age) groups. Because of the confusion this causes, the official geologic timescale of the ICS does not contain any of these names any longer.