function word vs closed-class word

function word and closed-class word both are nouns.

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

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

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

function word (noun)	closed-class 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