Celtic

The words Celt and Celtic can have many meanings.  

Pronounced /kel-(tik)/:

Ancient Celtic religion, a pagan religion.
Celt, either ancient or Modern Celts
Celtic languages, descending from the Proto-Celtic language, spoken by these people and their modern descendants
Celtic mythology
Celtici, Celts from ancient Lusitania (which is now Portugal). (See also Celtiberians for a Celtic people on the Iberian peninsula)
Irish music, folk music that started in "Celtic" cultures.
Celtic art, art from the "Celtic" culture
Celtic Christianity
Celtic calendar

The following should only be pronounced /selt(ik)/:

Boston Celtics professional basketball team
Celtic F.C. a football team in Glasgow