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 (see Oxford English Dictionary).
The word is believed to have originated in an early Continental Celtic language, but it comes to us from Greek (Keltoi), where it is spelled with a kappa; thus /k/ is the original pronunciation. This was borrowed into Latin (Celtae), where it was likewise pronounced /k/. 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. For additional discussion see Latin pronunciation.
Until the mid-20th century, Celtic was usually pronounced with /s/ in English except by academics, but the pronunciation with /k/ has been gaining ground rapidly. Following the usage of philologists, /k/ is now almost invariably used with reference to Celtic languages even in non-academic contexts. It is also the more popular pronunciation when talking about most other aspects of Celtic culture. However /s/ remains the only recognised pronunciation of the word when it occurs in the names of sports teams, most notably Celtic Football Club and the Boston Celtics basketball team; as these are proper names, the traditional pronunciation is entrenched.
There is a great deal of misinformation in circulation on this topic. It should be noted that there is no American-British distinction in these pronunciations nor is there a Scottish–Irish distinction. Neither pronunciation has been influenced by any modern Celtic languages, nor by Old Norse. The corresponding words in French are pronounced with /s/ while those in German have /k/, but neither French nor German has influenced English usage; rather, they show independent reflexes of the same phenomena in Latin and Greek.
The stone tool, a celt, has a completely separate etymology. In English it is pronounced only /sÉ›lt/.