ethnos vs ethnic group

ethnos and ethnic group both are nouns.

Word	Noun	Adjective	Verb	Adverb
ethnos	Yes	No	No	No
ethnic group	Yes	No	No	No

As nouns, ethnos and ethnic group are synonyms defined as:

ethnos and ethnic group: people of the same race or nationality who share a distinctive culture

ethnos (noun)	ethnic group (noun)
people of the same race or nationality who share a distinctive culture	people of the same race or nationality who share a distinctive culture