The English word is modern, attested from 1707 in the writings of Edward Lhuyd whose work, along with that of other late 17th century scholars, brought academic attention to the languages and history of these early inhabitants of Great Britain.[5]

Nowadays “Celt” and “Celtic” are usually pronounced /kɛlt/ and /kɛltɪk/, derived from a Greek root keltoi, when referring to the ethnic group and its languages. The pronunciation /’sɛltɪk/, derived from the French celtique, is mainly used for the names of sports teams (for example the NBA team, Boston Celtics and the SPL side, Celtic F.C. in Glasgow.

The first person to use the term “Celt” in relation to Britain and Ireland was George Buchanan in 1582. [citation needed] After its employment by Edward Lhuyd in 1707,[8] the use of the word “Celtic” as an umbrella term for the pre-Roman peoples of Britain gained considerable popularity in the nineteenth century, and remains in common usage. However its historical basis is now seen as dubious by many historians and archaeologists, and this usage has been called into question.

Simon James, formerly of the British Museum, in his book The Atlantic Celts: Ancient People or Modern Invention? makes the point that the Romans never used the term “Celtic” (or, rather, a cognate in Latin) in reference to the peoples of Britain and Ireland, and points out that the modern term “Celt” was coined as a useful umbrella term in the early 18th century to distinguish the non-English inhabitants of the archipelago when England united with Scotland in 1707 to create the Kingdom of Great Britain and the later union of Great Britain and Ireland as the United Kingdom in 1800. Nationalists in Scotland, Ireland and Wales looked for a way to differentiate themselves from England and assert their right to independence. James then argues that, despite the obvious linguistic connections, archeology does not suggest a united Celtic culture and that the term is misleading, no more (or less) meaningful than “Western”.

Where did you get that from ?

It comes from Greek – a son of Herakles called Keltus.