The name ‘Celt’ has come to refer to a group of people who speak Celtic languages — a branch of Indo-European languages that includes modern Irish, Scottish Gaelic, Welsh, Breton, Cornish and Manx. Those who speak these languages are mostly located on and around the islands of the United Kingdom and Ireland, though Celtic-speaking peoples once occupied most of Europe.
The modern idea of the Celts is one that did not exist at the height of Celtic societies. Instead, the Celts were a people who did not generally consider themselves as having a nation, and although the various groups of Celtic-speakers, such as the Gaels, Bretons and Gauls, all spoke similar languages, they did not generally consider themselves to have any relation to each other. Even those who spoke the same language did not have a sense of nationalism, but focused their identities on smaller units such as tribes and clans.
The word ‘Celt’ seems to come from the Greek κελτοι (keltoi), a word that was used to refer to a number of peoples from Europe. It is not clear whether the Greeks and Romans who used the term had an idea of the Celts as a single people, or if they simply used the term to refer to all foreign Europeans. The use of the term then fades away from history until more recently, when it appears as a 19th Century term to classify this linguistic group.
Because the Celts did not have a national sense of identity, and because their cultures developed in such diverse locations as Gaul (modern France), the Iberian peninsula and the British isles, there is a wide range of beliefs, legends and stories in Celtic mythology.