Meaning

noun a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times

originFrom Latin Celtae (singular Celta) from Ancient Greek Κελτοί (Keltoí). English Celts is from the 17th century. Until the mid 19th century, /sɛlt/ is the only recorded pronunciation. A consciously archaizing pronunciation /kɛlt/ was advocated during Irish and Welsh nationalism beginning in the 1850s.

Etymology from Wiktionary, CC BY-SA 3.0