The various names used since classical times for the people known today as the Celts are of disparate origins.
The names Κελτοί (Keltoí) and Celtae are used in Greek and Latin, respectively, to denote a people of the La Tène horizon in the region of the upper Rhine and Danube during the 6th to 1st centuries BC in Graeco-Roman ethnography. The etymology of this name and that of the Gauls Γαλάται Galátai / Galli is uncertain.
The Celts (/kɛlts/ KELTS, see pronunciation for different usages) or Celtic peoples (/ˈkɛltɪk/ KEL-tik) were a collection of Indo-European peoples in Europe and Anatolia, identified by their use of Celtic languages and other cultural similarities. Major Celtic groups included the Gauls; the Celtiberians and Gallaeci of Iberia; the Britons, Picts, and Gaels of Britain and Ireland; the Boii; and the Galatians. The interrelationships of ethnicity, language and culture in the Celtic world are unclear and debated; for example over the ways in which the Iron Age people of Britain and Ireland should be called Celts. In current scholarship, 'Celt' primarily refers to 'speakers of Celtic languages' rather than to a single ethnic group.
The history of pre-Celtic Europe and Celtic origins is debated. The traditional "Celtic from the East" theory, says the proto-Celtic language arose in the late Bronze Age Urnfield culture of central Europe, named after grave sites in southern Germany, which flourished from around 1200 BC. This theory links the Celts with the Iron Age Hallstatt culture which followed it (c. 1200–500 BC), named for the rich grave finds in Hallstatt, Austria, and with the following La Tène culture (c. 450 BC onward), named after the La Tène site in Switzerland. It proposes that Celtic culture spread westward and southward from these areas by diffusion or migration. A newer theory, "Celtic from the West", suggests proto-Celtic arose earlier, was a lingua franca in the Atlantic Bronze Age coastal zone, and spread eastward. Another newer theory, "Celtic from the Centre", suggests proto-Celtic arose between these two zones, in Bronze Age Gaul, then spread in various directions. After the Celtic settlement of Southeast Europe in the 3rd century BC, Celtic culture reached as far east as central Anatolia, Turkey.
The modern Celts (/kɛlts/ KELTS, see pronunciation of Celt) are a related group of ethnicities who share similar Celtic languages, cultures, genetics, and artistic histories, and who live in or descend from one of the regions on the western extremities of Europe populated by the Celts.
A modern Celtic identity emerged in Western Europe following the identification of the native peoples of the Atlantic fringe as Celts by Edward Lhuyd in the 18th century. Lhuyd and others (notably the 17th century Breton chronologist Pezron) equated the Celts described by Greco-Roman writers with the pre-Roman peoples of France, Great Britain, and Ireland. They categorised the ancient Irish and British languages as Celtic languages. The descendants of these ancient languages are the Brittonic (Breton, Cornish, and Welsh variants) and Goidelic (Irish, Manx, and Gaelic variants) languages, and the people who speak them are considered modern Celts.