It depends on what one means by the term Celt or Celtic. For linguists, the term Celts means speakers of one of the Celtic languages, either living (Irish, Scottish Gaelic, Manx, Welsh, Breton) or extinct (Cornish, Gaulish, Celtiberian, Lepontic, Galatian). These languages are or were located in the British Isles, northern Italy, France, Belgium, and the Iberian peninsula. Galatian was the one outlier in Asia Minor, but there is a controversy about whether the Celts there immigrated from west to east or east to west. Celt as an ethnic term is one used (borrowed or coined) by the ancient Greeks and Romans, and though based on contact with actual people, who may or may not have spoken a Celtic language, soon become something of a synonym for barbarian. The term in archeology is associated with a wide-spread (in Europe that is) collection of cultural artifacts (La Tène, Hallstatt, etc.) that may or may not have been produced by Celtic-speaking peoples. It is this identification of certain iron-age and bronze-age cultures that is coming under scrutiny in the books I mentioned in my previous posting to this thread.

Ceci n'est pas un seing.