During the Iron Age, Celtic culture was spread from the Iberian Peninsula to Anatolia (Turkey), but the ultimate origin of the Celts is a subject of controversy. Traditionally, scholars have placed the Celtic homeland in what is now southern Germany and Austria, associating the earliest Celtic peoples with the Hallstatt culture. (see List of Celtic tribes for the names of specific, early Celtic peoples).

Although more recently restricted to the Atlantic coast of Western Europe (known as the 'Celtic fringe'), Celtic languages were once predominant over much of Europe, with territory largely ceded to expanding Germanic tribes and the invading Roman Empire. Archaeological and historical sources show that at their maximum extent in the third century BC, Celtic peoples were also present in areas of Eastern Europe and Asia Minor.^[2]

'Celticity' generally refers to the cultural commonalities of these peoples, based on similarities in language, material artifacts, social organisation and mythological factors. Earlier theories were that this indicated a common racial origin but more recent theories are reflective of culture and language rather than race. Celtic cultures seem to have had numerous diverse characteristics but the commonality between these diverse peoples was the use of a Celtic language.

'Celtic' is a descriptor of a family of languages and, more generally, means 'of the Celts,' or 'in the style of the Celts'. It has also been used to refer to several archaeological cultures defined by unique sets of artifacts. The link between language and artifact is aided by the presence of inscriptions. (see Celtic (disambiguation) for other applications of the term)

Today, the term 'Celtic' is generally used to describe the languages and respective cultures of Ireland, Scotland, Wales, Cornwall, the Isle of Man and Brittany, also known as the Six Celtic Nations. These are the regions where four Celtic languages are still spoken to some extent as mother tongues: Irish Gaelic, Scottish Gaelic, Welsh, and Breton plus two recent revivals, Cornish (one of the Brythonic languages) and Manx (one of the Goidelic languages). 'Celtic' is also sometimes used to describe regions of Continental Europe that have Celtic heritage, but where no Celtic language has survived; these areas include the northern Iberian Peninsula (northern Portugal, and the Spanish historical regions of Galicia, Asturias and Cantabria), and to a lesser degree, France. (see Modern Celts)

'Continental Celts' refers to the Celtic-speaking people of mainland Europe. 'Insular Celts' refers to the Celtic-speaking people of the British isles and their descendants. The Celts of Brittany derive their language from migrating insular Celts from the British Isles and so are grouped accordingly.