The Celts were a collection of Indo-European peoples in Europe and Anatolia, identified by their use of Celtic languages and other cultural similarities. They were never politically united as a single people but consisted of different groups, including Gauls and Celtiberians, who spoke different languages that might not have been mutually understood. The term "Celtic" refers to a language family and, more generally, means "of the Celts" or "in the style of the Celts".

The earliest surviving references to the Celts were recorded by Greek geographer Hecataeus of Miletus in the sixth century B.C., and in the centuries that followed, "the Celts are mentioned with increasing frequency by Greek and Roman writers". The Celts inhabited a vast area of continental Europe as far east as modern-day Turkey.

The modern idea of a Celtic cultural identity or "Celticity" focuses on similarities among languages, works of art, and classical texts, and sometimes also among material artifacts, social organization, homeland, and mythology. Today, the term Celtic generally refers to the languages and cultures of Ireland, Scotland, Wales, Cornwall, the Isle of Man, and Brittany; also called the Celtic nations. These are the regions where Celtic languages are still spoken to some extent. The six surviving Celtic languages are Irish, Scottish Gaelic, Welsh, Breton, Cornish, and Manx.