The term Celt, normally pronounced /kɛlt/ (see below), refers to a member of any of a number of peoples in Europe using the Celtic languages, which form a branch of Indo-European languages, as well as others whose language is unknown but where associated cultural traits such as Celtic art are found in archaeological evidence. Historical theories were developed that these factors were indicative of a common origin, but later theories of culture spreading to differing indigenous peoples have recently been supported by some genetic studies.

The Celts themselves had an intricate, indigenous polytheistic religion and distinctive culture, though the spread of the Roman Empire led to continental Celts adopting Roman culture. The eventual development of Celtic Christianity in Ireland and Britain brought an early medieval renaissance of Celtic art between 400 and 1200, only ended by the Norman Conquest of Ireland in the late 12th century.