noun vs open-class word

noun and open-class word both are nouns.

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

As nouns, open-class word is a hypernym of noun; that is, open-class word is a word with a broader meaning than noun:

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

Other hypernyms of noun include content word, substantive.

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