The Belgae (ˈbɛldʒiː,_ˈbɛlɡaɪ) were a large confederation of tribes living in northern Gaul, between the English Channel, the west bank of the Rhine, and the northern bank of the river Seine, from at least the third century BC. They were discussed in depth by Julius Caesar in his account of his wars in Gaul. Some peoples in southern Britain were also called Belgae and had apparently moved from the continent. T. F. O'Rahilly believed that some had moved further west and he equated them with the Fir Bolg in Ireland. The Roman province of Gallia Belgica was named after the continental Belgae. The term continued to be used in the region until the present day and is reflected in the name of the modern country of Belgium.
The consensus among linguists is that the ethnic name Belgae probably comes from the Proto-Celtic root *belg- or *bolg- meaning "to swell (particularly with anger/battle fury/etc.)", cognate with the Dutch adjective gebelgd "very angry" (weak perfect participle of the verb belgen "to become angry") and verbolgen "being angry" (strong perfect participle of obsolete verbelgen "to make angry"), as well as the Old English verb belgan, "to be angry" (from Proto-Germanic *balgiz), derived ultimately from the Proto-Indo-European root *bhelgh- ("to swell, bulge, billow"). Thus, a Proto-Celtic ethnic name *Bolgoi could be interpreted as "the people who swell (particularly with anger/battle fury)".
In Caesar's usage, Belgium is a geographical subregion comprising the Bellovaci, Ambiani, Atrebates, and Veromandui. These four communities are widely thought to have been the original Belgae in Gaul.
Julius Caesar describes Gaul at the time of his conquests (58–51 BC) as divided into three parts, inhabited by the Aquitani in the southwest, the Gauls of the biggest central part, who in their own language were called Celtae, and the Belgae in the north. Each of these three parts, he says, differed in terms of customs, laws and language.