The terms Celt and Celtic were originally used by ancient Greek and Roman writers to refer to an extensive network of tribes located primarily in Gaul (roughly modern-day France, Belgium, and northern Italy) who claimed, or were thought by their neighbors, to share a common descent. These terms, however, were never used in reference to the peoples of Britain and Ireland, even though it is now known that they did (and some still do) speak Celtic languages. Some classical writers did note traits common to both the Celts and the Britons, such as the institution of druids and druidism, which, according to Caesar, originated in Britain. The use of the ethnonym Celtic to refer to related languages both modern and ancient (that in turn constitute a subset of the Indo-European family of languages) dates back to the eighteenth century, arising in the wake of...