Edward Lhuyd in the context of Celtic language family

The Celtic languages (/ˈkɛltɪk/ KEL-tik) are a branch of the Indo-European language family, descended from the hypothetical Proto-Celtic language. The term "Celtic" was first used to describe this language group by Edward Lhuyd in 1707, following Paul-Yves Pezron, who made the explicit link between the Celts described by classical writers and the Welsh and Breton languages.

During the first millennium BC, Celtic languages were spoken across much of Europe and central Anatolia. Today, they are restricted to the northwestern fringe of Europe and a few diaspora communities. There are six living languages: the four continuously living languages Breton, Irish, Scottish Gaelic and Welsh, and the two revived languages Cornish and Manx. All are minority languages in their respective countries, though there are continuing efforts at revitalisation. Welsh is an official language in Wales and Irish is an official language across the island of Ireland and of the European Union. Welsh is the only Celtic language not classified as endangered by UNESCO. The Cornish and Manx languages became extinct in modern times but have been revived. Each now has several hundred second-language speakers.

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.