Learn how to say Celticism correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word Celtic:
relating to the Celts or their languages, which constitute a branch of the Indo-European family and include Irish, Scottish Gaelic, Welsh, Breton, Manx, Cornish, and several extinct pre-Roman languages such as Gaulish.
the Celtic language group. See also P-Celtic, Q-Celtic.
late 16th century: from Latin Celticus (from Celtae ‘Celts’), or from French Celtique (from Celte ‘Breton’)