earliest and lowermost division of Miocene (Miocene Epoch) rocks, representing all rocks deposited worldwide during the Aquitanian Age (23 million to 20.4 million years ago) of the Neogene Period (the past 23 million years). The stage is named for exposures in the region of Aquitaine in southwestern France.

The Global Stratotype Section and Point (GSSP) defining the lower boundary of this stage, ratified by the International Commission on Stratigraphy (ICS) in 1996, is located in the Lemme-Carrosio Section within the village of Carrosio, Italy. This lower boundary closely coincides with the base of the zone of the foraminiferan (pseudopod-using unicellular organism protected by a test or shell) Paragloborotalia kugleri and the first occurrence of the calcerous nannofossil (remains of ocean-dwelling golden-brown algae) Sphenolithus capricornutus. The Aquitanian Stage precedes the Burdigalian Stage and is underlain by the Chattian Stage of the Paleogene Period.