Celtic
Pronunciation: /ˈkɛltɪk, ˈs-/
adjective

Origin:
late 16th century: from Latin Celticus (from Celtae 'Celts'), or from French Celtique (from Celte ‘Breton’)

Celt and Celtic can be pronounced either with an initial k- or s-, but in standard English the normal pronunciation is with a k-, except in the name of the Glaswegian football club.