fifth of six main divisions (in ascending order) in the Lower Cretaceous Series, representing rocks deposited worldwide during the Aptian Age, which occurred 125 to 112 million years ago during the Cretaceous Period. Rocks of the Aptian Stage overlie those of the Barremian Stage and underlie rocks of the Albian Stage.

The name of the stage is derived from the town of Apt in Vaucluse département in southeastern France, for which the surrounding area serves as the classic type district for rocks of this age. In Britain the Aptian Stage is represented by part of the Lower Greensand formation. Elsewhere in northern Europe it consists of portions of the thick Hils clay, while in the United States it includes the Dakota Sandstone. The ammonite (ammonoid) genus Prodeshayesites is used as an index fossil to mark the base of the Aptian Stage in Britain, Germany, and France. The Aptian has been divided into several shorter spans of time called biozones (biozone), some of which are characterized by calcareous nannofossils of Nannoconus bucheri and N. wassalli. The planktonic foraminiferans (foraminiferan) Globigerinelloides algerianus and G. blowi are also considered index fossils for rocks of this stage.