Gaul (Latin: Gallia) was the name given, in ancient times, to the region of Western Europe comprising present-day northern Italy, France, Belgium, western Switzerland and the parts of the Netherlands and Germany on the west bank of the Rhine river, what should become part of the Frankish Empire.
In English, the word Gaul (French: Gaulois) may also refer to a Celtic inhabitant of that region, although the expression may be used more generally for all ancient speakers of the Gaulish language (a derivative of early Celtic) who were widespread in Europe (Germania) and extended even into central Anatolia by Roman times. In this way, "Gaul" and "Celt" are sometimes used interchangeably.
Gauls under Brennus sacked Rome circa 390 BC. In the Aegean world, a huge migration of Eastern Gauls appeared in Thrace, north of Greece, in 281 BC. Another Gaulish chieftain named Brennus, at the head of a large army, was only turned back from desecrating the Temple of Apollo at Delphi at the last minute — he was alarmed, it was said, by portents of thunder and lightning. At the same time a migrating band of Celts, some 10,000 warriors, with their women and children and slaves, were moving through Thrace. Three tribes of Gauls crossed over from Thrace to Asia Minor at the express invitation of Nicomedes I, king of Bithynia, who required help in a dynastic struggle against his brother. Eventually they settled down in eastern Phrygia and Cappadocia in central Anatolia, a region henceforth known as Galatia.