During the first century AD, the languages used in Britain were Brythonic by the native tribal peoples and Latin by the Roman invaders. In The Druid Chronicles I’ve used words not in common usage in the English language until the 1500s and later, on the reasoning these peoples had words of similar meaning in their own languages at that time.
It was likely the Romans who called the ancient peoples of Europe and Britain Celts. They would have called themselves by their own tribal names. For clarity, I have taken the liberty of using the term “Celt” in reference to the ancient tribal peoples of Cymru as a whole.