The pronunciation of the words Celt and Celtic in their various meanings has been surrounded by some confusion: the initial <c> can be realised either as /k/ or as /s/. Both can be justified philologically and both are "correct" in terms of English prescriptive usage.
The word presumably originated in an Early Continental Celtic language, but it comes to us from Greek, where it is spelled with a kappa; thus /k/ is the original pronunciation. However in Mediaeval Latin, the letter <c>, originally pronounced /k/, shifted to /s/, a process known as palatalization, and many words and names borrowed from Latin into English after this sound shift are pronounced this way: centre, Cicero, et cetera. Thus /s/ is the inherited pronunciation in English.
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