Origins
The word 'Celtic' enters English through Latin 'Celtae' and ultimately from Greek 'Keltoí' (Κελτοί), one of the earliest ethnonyms in European history. The Greek historian Herodotus, writing around 450 BCE, is the first known author to use the term, locating the Keltoi near the source of the Danube and beyond the Pillars of Heracles (the Strait of Gibraltar). Subsequent Greek and Roman writers — Polybius, Strabo, Caesar, Diodorus Siculus — used 'Keltoi' or 'Celtae' for the vast constellation of related peoples stretching from Iberia and Ireland to Galatia in Asia Minor.
The origin of the name is disputed. The most widely discussed hypothesis connects it to a Proto-Celtic self-designation *keltos, possibly related to the PIE root *kel- (to hide, to conceal), which also produced Latin 'cēlāre' (to hide), the source of English 'conceal.' Under this interpretation, the Celts may have been 'the hidden ones' — perhaps forest-dwellers, or perhaps wielders of secret knowledge (a connection to druidic tradition has been suggested but cannot be proved). An alternative proposal links the name to PIE *kelh₂- (to strike, to cut), producing a meaning of 'the strikers' or 'the fighters' — appropriate for peoples whose warrior aristocracy was legendary in the ancient world.
The Romans drew a distinction between 'Celtae' (used for the peoples of Gaul) and 'Galli' (also used for Gauls, and the source of 'Gaul' and 'Gaelic'). Caesar famously opens 'De Bello Gallico' with the statement that Gaul is divided into three parts, inhabited by the Belgae, the Aquitani, and 'those who in their own language are called Celtae, in our language Galli.' This passage suggests that 'Celtae' was indeed a native self-designation, at least for some of the Gaulish peoples.
Latin Roots
The word largely disappeared from European discourse during the Middle Ages, when the Celtic-speaking peoples were known by their individual national names — Irish, Welsh, Breton, Scottish. It was the Welsh antiquarian Edward Lhuyd who revived the term in his 'Archaeologia Britannica' (1707), demonstrating through systematic comparison that Irish, Welsh, Cornish, Breton, Scottish Gaelic, and Manx were related languages forming a distinct family. Lhuyd's work established 'Celtic' as a linguistic term, paralleling the contemporaneous use of 'Germanic' and 'Romance' for other European language groups.
The Celtic language family is conventionally divided into two branches. The Goidelic (or Q-Celtic) branch includes Irish, Scottish Gaelic, and Manx, characterized by retaining the Proto-Celtic *kw sound as a velar (Irish 'ceathair' for 'four'). The Brythonic (or P-Celtic) branch includes Welsh, Cornish, and Breton, in which *kw became /p/ (Welsh 'pedwar' for 'four'). This division has been debated — some scholars prefer a division into Insular Celtic (the island languages) versus Continental Celtic (the extinct languages of Gaul, Celtiberia, Galatia, and Lepontic northern Italy) — but the Goidelic/Brythonic split remains the most commonly used framework.
The pronunciation of 'Celtic' itself encodes a linguistic history. The Greek Κ (kappa) in 'Keltoí' was unambiguously a /k/ sound, and the hard-C pronunciation /ˈkɛl.tɪk/ is the historically grounded form. The soft-C pronunciation /ˈsɛl.tɪk/ arose in English around the seventeenth century, when scholars unfamiliar with the Greek original treated the word as if it followed the Latin/French pattern of softening C before E and I. Both pronunciations coexist in modern English, with /ˈkɛl.tɪk/ preferred in academic and linguistic contexts and /ˈsɛl.tɪk/ entrenched in popular culture, most notably through the Boston Celtics basketball team (founded 1946).
Modern Legacy
The cultural resonance of 'Celtic' extends far beyond linguistics. The nineteenth-century Celtic Revival — led by figures like W.B. Yeats, Lady Gregory, and Douglas Hyde — reclaimed Celtic identity as a source of artistic and national inspiration, particularly in Ireland and Scotland. Today, 'Celtic' evokes a cultural complex that includes knotwork art, druidism, the Irish and Welsh literary traditions, Celtic music, and a distinctive spiritual sensibility — though historians caution that much of what is popularly called 'Celtic' reflects modern romanticism more than ancient reality.