Gallia Celtica in the context of "Aquitani"

Gaul (Latin: Gallia) was a region of Western Europe first clearly described by the Romans, encompassing present-day France, Belgium, Luxembourg, and parts of Switzerland, the Netherlands, Germany, and Northern Italy. It covered an area of 494,000 km (191,000 sq mi). According to Julius Caesar, who took control of the region on behalf of the Roman Republic, Gaul was divided into three parts: Gallia Celtica, Belgica, and Aquitania. Archaeologically, the Gauls were bearers of the La Tène culture during the 5th to 1st centuries BC. This material culture was found throughout Gaul and as far east as modern-day southern Poland, Slovakia, and Hungary.

Warbands led by the Gaul Brennos sacked Rome in 387 BC, becoming the only time Rome was conquered by a foreign enemy in 800 years. However, Gallia Cisalpina was conquered by the Romans in 204 BC and Gallia Narbonensis in 123 BC. Gaul was invaded after 120 BC by the Cimbri and the Teutons, who were in turn defeated by the Romans by 103 BC. Julius Caesar finally subdued the largest part of Gaul in his campaigns from 58 to 51 BC. Roman control of Gaul lasted for five centuries, until the last Roman rump state, the Domain of Soissons, fell to the Franks in AD 486.

Gallia Lugdunensis (French: Gaule Lyonnaise) was a province of the Roman Empire in what is now the modern country of France, part of the Celtic territory of Gaul formerly known as Celtica. It is named after its capital Lugdunum (today's Lyon), possibly Roman Europe's major city west of Italy, and a major imperial mint. Outside Lugdunum was the Sanctuary of the Three Gauls, where representatives met to celebrate the cult of Rome and Augustus.

The Gallo-Brittonic languages, also known as the P-Celtic languages, are a proposed subdivision of the Celtic languages containing the languages of Ancient Gaul (both Celtica and Belgica) and Celtic Britain, which share certain features. Besides common linguistic innovations, speakers of these languages shared cultural features and history. The cultural aspects are commonality of art styles and worship of similar gods. Coinage just prior to the British Roman Period was also similar. In Julius Caesar's time, the Atrebates held land on both sides of the English Channel.

It contrasts with the Insular Celtic hypothesis, which asserts that Goidelic and Brythonic underwent a period of common development and have shared innovations to the exclusion of Gaulish, while the shared changes are either independent innovations that occurred separately in Brythonic and Gaulish or are due to language contact between the two groups.