At the time of the Roman conquest, Julius Caesar, who defeated them in his campaign in Gaul, describes them as making up a distinct part of Gaul:
All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in ours Gauls, the third. All these differ from each other in language, customs and laws. The river Garonne separates the Gauls from the Aquitani
Despite apparent cultural and linguistic connections to Iberia (Vascones) and to Iberians, the area of Aquitania, as a part of Gaul ended at the Pyrenees according to Caesar:
Aquitania extends from the river Garonne to the Pyrenaean mountains and to that part of the ocean which is near Spain: it looks between the setting of the sun, and the north star.
Relation to Basque people and language
The presence, on late Romano-Aquitanian funerary slabs, of what seem to be the names of deities or people similar to certain names in modern Basque have led many philologists and linguists to conclude that Aquitanian was closely related to an older form of Basque. Julius Caesar draws a clear line between the Aquitani, living in present-day south-western France and speaking Aquitanian, and their neighboring Celts living to the north. The fact that the region was known as Vasconia in the Early Middle Ages, a name that evolved into the better known form of Gascony, along with other toponymic evidence, seems to corroborate that assumption.
Tribes in Aquitania (as was defined in the 1st century BCE)
Late distribution of tribes in Novempopulania at the end of the 6th century CE, former Aquitania proper (as was defined in the 1st century BCE)
Although the country where the original Aquitanians lived came to be named Novempopulania (nine peoples) in the late years of the Roman Empire and Early Middle Ages (up to the 6th century), the number of tribes varied (about 20 for Strabo, but comparing with the information of other classical authors such as Pliny, Ptolemy and Julius Caesar, the total number were 32 or 33):
^Waldman, Carl; Mason, Catherine (2006). Encyclopedia of European Peoples. Infobase Publishing. p. 38. ISBN9781438129181. The Aquitani [...] lived in Gaul in the region between the Garonne River and the Pyrenees in present-day southwestern France [...].
^These are indeed the opening lines of Caesar's account of his war in Gaul: Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam qui ipsorum lingua Celtae, nostra Galli appellantur. Hi omnes lingua, institutis, legibus inter se differunt. Gallos ab Aquitanis Garumna flumen [...] dividit. Julius Caesar, De bello Gallico 1.1, edition of T. Rice Holmes
^Aquitania a Garumna flumine ad Pyrenaeos montes et eam partem Oceani quae est ad Hispaniam pertinet; spectat inter occasum solis et septentriones.