The Celts ( /ˈkɛlts/ or /ˈsɛlts/, see pronunciation of Celtic) were a diverse group of tribal societies in Iron Age and Roman-era Europe who spoke Celtic languages.
The earliest archaeological culture commonly accepted as Celtic, or rather Proto-Celtic, was the central European Hallstatt culture (c. 800-450 BC), named for the rich grave finds in Hallstatt, Austria. By the later La Tène period (c. 450 BC up to the Roman conquest), this Celtic culture had expanded over a wide range of regions, whether by diffusion or migration: to the British Isles (Insular Celts), France and The Low Countries (Gauls), much of Central Europe, the Iberian Peninsula (Celtiberians, Celtici and Gallaeci) and northern Italy (Golaseccans and Cisalpine Gauls) and following the Gallic invasion of the Balkans in 279 BC as far east as central Anatolia (Galatians).
The earliest directly attested examples of a Celtic language are the Lepontic inscriptions, beginning from the 6th century BC. Continental Celtic languages are attested only in inscriptions and place-names. Insular Celtic is attested from about the 4th century AD in ogham inscriptions, although it is clearly much earlier. Literary tradition begins with Old Irish from about the 8th century. Coherent texts of Early Irish literature, such as the Táin Bó Cúailnge (The Cattle Raid of Cooley), survive in 12th-century recensions.
By mid 1st millennium AD, following the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture and Insular Celtic had become restricted to Ireland, to the western and northern parts of Great Britain (Wales, Scotland, Cornwall and the Isle of Man), and to northern France (Brittany). Between the fifth and eighth centuries AD the Celtic-speaking communities of the Atlantic regions had emerged as a reasonably cohesive cultural entity. In language, religion, and art they shared a common heritage that distinguished them from the culture of surrounding polities. The Continental Celtic languages ceased to be widely used by the 6th century.
Insular Celtic culture diversified into that of the Gaels (Irish, Scottish and Manx) and the Brythonic Celts (Welsh, Cornish, and Bretons) of the medieval and modern periods. A modern "Celtic identity" was constructed in the context of the Romanticist Celtic Revival in Great Britain, Ireland, and other European territories, such as Galicia. Today Irish, Scottish Gaelic, Welsh, and Breton remain spoken in parts of their historical territories, and both Cornish and Manx are currently undergoing revival.