open-class word vs noun

open-class word and noun both are nouns.

Word	Noun	Adjective	Verb	Adverb
open-class word	Yes	No	No	No
noun	Yes	No	No	No

As nouns, noun is a hyponym of open-class word; that is, noun is a word with a more specific, narrower meaning than open-class word:

open-class word: a word to which an independent meaning can be assigned
noun: a content word that can be used to refer to a person, place, thing, quality, or action

Other hyponyms of open-class word include head word, headword, verb, modifier, qualifier, numeral.

open-class word (noun)	noun (noun)
a word to which an independent meaning can be assigned	a content word that can be used to refer to a person, place, thing, quality, or action
	the word class that can serve as the subject or object of a verb, the object of a preposition, or in apposition