The Celts ( , see pronunciation for different usages) or Celtic peoples ( ) were a collection of Indo-European peoples. "The Celts, an ancient Indo-European people, reached the apogee of their influence and territorial expansion during the 4th century BC, extending across the length of Europe from Britain to Asia Minor."; . "[T]he Celts, were Indo-Europeans, a fact that explains a certain compatibility between Celtic, Roman, and Germanic mythology."; . "The Celts and Germans were two Indo-European groups whose civilizations had some common characteristics."; . "Celts and Germans were of course derived from the same Indo-European stock."; . "Celt, also spelled Kelt, Latin Celta, plural Celtae, a member of an early Indo-European people who from the 2nd millennium bce to the 1st century bce spread over much of Europe." in Europe and Anatolia, identified by their use of Celtic languages and other cultural similarities.. "Celts, a name applied by ancient writers to a population group occupying lands mainly north of the Mediterranean region from Galicia in the west to Galatia in the east. (Its application to the Welsh, the Scots, and the Irish is modern.) Their unity is recognizable by common speech and common artistic traditions.. "Celts, in its modern usage, is an encompassing term referring to all Celtic-speaking peoples.". "Celt, also spelled Kelt, Latin Celta, plural Celtae, a member of an early Indo-European people who from the 2nd millennium bce to the 1st century bce spread over much of Europe. Their tribes and groups eventually ranged from the British Isles and northern Spain to as far east as Transylvania, the Black Sea coasts, and Galatia in Anatolia and were in part absorbed into the Roman Empire as Britons, Gauls, Boii, Galatians, and Celtiberians. Linguistically they survive in the modern Celtic speakers of Ireland, Highland Scotland, the Isle of Man, Wales, and Brittany. Major Celtic groups included the Gauls; the Celtiberians and Gallaeci"If, as is the first criterion of this Encyclopedia, one bases the concept of 'Celticity' on language, one can apply the term 'Celtic' to ancient Galicia", 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, not 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 (–500 BC), named for the rich grave finds in Hallstatt, Austria, and with the following La Tène culture ( 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 earliest undisputed examples of Celtic language are the Lepontic inscriptions from the 6th century BC. Continental Celtic languages are attested almost exclusively through inscriptions and place-names. Insular Celtic languages are attested from the 4th century AD in Ogham inscriptions, though they were being spoken much earlier. Celtic literary tradition begins with Old Irish texts around the 8th century AD. Elements of Celtic mythology are recorded in early Irish and early Welsh literature. Most written evidence of the early Celts comes from Greco-Roman writers, who often grouped the Celts as barbarian tribes. They followed an ancient Celtic religion overseen by druids.

The Celts were often in conflict with the Romans, such as in the Roman–Gallic wars, the Celtiberian Wars, the conquest of Gaul and conquest of Britain. By the 1st century AD, most Celtic territories had become part of the Roman Empire. By , due to Romanisation and the migration of Germanic tribes, Celtic culture had mostly become restricted to Ireland, western and northern Britain, and Brittany. Between the 5th and 8th centuries, the Celtic-speaking communities in these Atlantic regions emerged as a reasonably cohesive cultural entity. They had a common linguistic, religious and artistic heritage that distinguished them from surrounding cultures.

Insular Celtic culture diversified into that of the Gaels (Irish, Scots and Manx) and the Celtic Britons (Welsh, Cornish, and Bretons) of the medieval and modern periods. A modern Celtic identity. "CELTS location: Greater Europe time period: Second millennium B.C.E. to present ancestry: Celtic was constructed as part of the Romanticist Celtic Revival in Britain, Ireland, and other European territories such as Galicia. Today, Irish, Scottish Gaelic, Welsh, and Breton are still spoken in parts of their former territories, while Cornish and Manx are undergoing a revival.

Names and terminology

Ancient The first recorded use of the name 'Celts' – as () in Ancient Greek – was by Greek geographer Hecataeus of Miletus in 517 BC,Sarunas Milisauskas, when writing about a people living near Massilia (modern Marseille), southern Gaul. In the fifth century BC, Herodotus referred to living around the source of the Danube and in the far west of Europe.Herodotus, The Histories, 2.33; 4.49. The etymology of is unclear. Possible roots include PIE verb *kʲel or *4k̑el- 'to hide' (seen also in Old Irish , and Modern Welsh ), *kʲel 'to heat' or *kel 'to impel'.John T. Koch (ed.), Celtic Culture: a historical encyclopedia. 5 vols. 2006. Santa Barbara, California: ABC-CLIO, p. 371. It may come from the proto-Celtic *kel-o-. Celt- is found in the names of several ancient Gauls such as Celtillus, father of Vercingetorix. Linguist Kim McCone (2006) suggests that Keltoi meant the people or 'descendants of the hidden one', noting that the Gauls claimed descent from an underworld deity (the Gaulish Dis Pater according to Caesar in De Bello Gallico). McCone links this underworld god with the Germanic Hel.McCone, Kim (2013). "The Celts: questions of nomenclature and identity", in Ireland and its Contacts. University of Lausanne. pp.21–27 Others view it as a name coined by Greeks; among them linguist Patrizia de Bernardo Stempel, who suggests it meant "the tall ones".P. De Bernardo Stempel 2008. "Linguistically Celtic ethnonyms: towards a classification", in Celtic and Other Languages in Ancient Europe, J. L. García Alonso (ed.), 101–18. Ediciones Universidad Salamanca.

In the first century BC, Roman leader Julius Caesar reported that the Gauls called themselves 'Celts', , in their own tongue.Julius Caesar, Commentarii de Bello Gallico 1.1: "All Gaul is divided into three parts, one of which the Belgae live, another in which the Aquitani live, and the third are those who in their own tongue are called , in our language ." Thus, whether or not it was given to them by others, the name 'Celt' was used by the Celts themselves. Greek geographer Strabo, writing about Gaul towards the end of the first century BC, refers to the "race which is now called both Gallic and Galatic", though he also uses Celtica as another name for Gaul. He reports Celtic peoples in Iberia too, calling them Celtiberi and Celtici.Strabo, Geography, 3.1.3; 3.1.6; 3.2.2; 3.2.15; 4.4.2. Pliny the Elder noted the use of Celtici in Lusitania as a tribal surname,Pliny the Elder, The Natural History 21: "the Mirobrigenses, surnamed Celtici" ("Mirobrigenses qui Celtici cognominantur"). which epigraphic findings have confirmed.Fernando De Almeida, Breve noticia sobre o santuário campestre romano de Miróbriga dos Celticos (Portugal): D(IS) M(ANIBUS) S(ACRUM) / C(AIUS) PORCIUS SEVE/RUS MIROBRIGEN(SIS) / CELT(ICUS) ANN(ORUM) LX / H(IC) S(ITUS) E(ST) S(IT) T(IBI) T(ERRA) L(EVIS).

A Latin name for the Gauls, (), may come from a Celtic ethnic name, perhaps borrowed into Latin during the Celtic expansion into Italy from the early fifth century BC. Its root may be Proto-Celtic *galno, meaning "power, strength" (whence Old Irish gal "boldness, ferocity", Welsh gallu "to be able, power"). The Greek name Γαλάται (, Latinized Galatae) most likely has the same origin, referring to the Gauls who invaded southeast Europe and settled in Galatia. The suffix -atai might be a Greek inflection. Linguist Kim McCone suggests it comes from Proto-Celtic *galatis ("ferocious, furious"), and was not originally an ethnic name but a name for young warrior bands. He says "If the Gauls' initial impact on the Mediterranean world was primarily a military one typically involving fierce young *galatīs, it would have been natural for the Greeks to apply this name for the type of Keltoi that they usually encountered".

Because most Classical writers did not call the inhabitants of Britain and Ireland () or , some scholars prefer not to call them 'Celts' either. However, it is acknowledged that they were 'Celtic' because they spoke Celtic languages and shared other cultural traits with Celtic peoples. Roman historian Tacitus, for example, says the Britons resembled the Gauls in customs and religion.

Modern For at least 1,000 years, the ethnonym Celt is largely absent from textual sources. An exception is found in the writings of Byzantine historians of the 11th to 13th centuries, including Michael Psellos, Anna Komnene and Niketas Choniates, who use the term, Keltoi, in reference to Western Europeans, especially Normans and the French, and sometimes the English. This was done as a self-conscious archaism, harking back to ethnonyms used in Classical writings, in the same way Turks were called Persians and Russians 'Tauroscyths'.Psellus (trans, Sewter), p.45Choniates (trans, Magoulias), pp. 17, 147 However, no ethnic group called themselves Celts or Celtic again until about 1700. After the word 'Celtic' was rediscovered in classical texts, it was applied for the first time to the distinctive culture, history, traditions, and language of the modern Celtic nations – Ireland, Scotland, Wales, Cornwall, Brittany, and the Isle of Man.Who were the Celts? It's complicated , Irish Times, Denis Staunton, 20 October 2015, accessed 17 April 2023. 'Celt' is a modern English word, first attested in 1707 in the writing of Edward Lhuyd, whose work, along with that of other late 17th-century scholars, brought academic attention to the languages and history of the early Celtic inhabitants of Great Britain.Lhuyd, E. Archaeologia Britannica; An account of the languages, histories, and customs of the original inhabitants of Great Britain. (reprint ed.) Irish University Press, 1971, p. 290. . The English words Gaul, Gauls () and Gaulish (first recorded in the 16–17th centuries) come from French and , a borrowing from Frankish *, 'Roman-land' , the root of which is Proto-Germanic , 'foreigner, Roman, Celt', whence the English word Welsh (Old English ). Proto-Germanic comes from the name of the Volcae, a Celtic tribe who lived first in southern Germany and central Europe, then migrated to Gaul. This means that English Gaul, despite its superficial similarity, is not actually derived from Latin (which should have produced * in French), though it does refer to the same ancient region.