closed-class word vs function word

closed-class word and function word both are nouns.

Word	Noun	Adjective	Verb	Adverb
closed-class word	Yes	No	No	No
function word	Yes	No	No	No

As nouns, closed-class word and function word are synonyms defined as:

closed-class word and function word: a word that is uninflected and serves a grammatical function but has little identifiable meaning

closed-class word (noun)	function word (noun)
a word that is uninflected and serves a grammatical function but has little identifiable meaning	a word that is uninflected and serves a grammatical function but has little identifiable meaning