Celt vs European

Celt and European both are nouns.

Word	Noun	Adjective	Verb	Adverb
Celt	Yes	No	No	No
European	Yes	Yes	No	No

As nouns, European is a hypernym of Celt; that is, European is a word with a broader meaning than Celt:

Celt: a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times
European: a native or inhabitant of Europe

Celt (noun)	European (noun)
a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times	a native or inhabitant of Europe

Celt (adjective)	European (adjective)
	of or relating to or characteristic of Europe or the people of Europe