Celt vs Briton

Celt and Briton both are nouns.

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

As nouns, Briton is a hyponym of Celt; that is, Briton is a word with a more specific, narrower meaning than Celt:

Celt: a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times
Briton: an inhabitant of southern Britain prior to the Anglo-Saxon invasions

Other hyponyms of Celt include Gael, Gaul.

Celt (noun)	Briton (noun)
a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times	an inhabitant of southern Britain prior to the Anglo-Saxon invasions
	a native or inhabitant of Great Britain

Celt (adjective)	Briton (adjective)
	characteristic of or associated with the Britons