The Rhaetian is named after the Rhaetian Alps, a mountain chain stretching over parts of eastern Switzerland, northern Italy and western Austria. The stage was introduced in scientific literature by Austrian geologist Eduard Suess and German paleontologist Albert Oppel in 1856.
The base of the Rhaetian did not yet have a unanimously agreed upon definition in 2009. In the Tethyan domain, the base of the ammonite biozone of
Sagenites reticulatus is used, in the
boreal domain (where this species is not found) the base of the biozone of
Cochloceras amoenum is used instead. The base is also close to the first appearances of conodont species Misikella spp. and Epigondolella mosheri and radiolarite species
The top of the Rhaetian (the base of the Hettangian stage, the Lower Jurassic series and the Jurassic system) is at the first appearance of ammonite genus Psiloceras.
In the Tethyan domain, the Rhaetian contains two ammonite biozones. The highest ammonite biozone is that of
Choristoceras marshi, the lower one that of
Rhabdoceras suesii. The end of this period is marked by the Triassic-Jurassic extinction event .