head word vs open-class word

head word and open-class word both are nouns.

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

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

Other hypernyms of head word include content word.

head word (noun)	open-class word (noun)
(grammar) the word in a grammatical constituent that plays the same grammatical role as the whole constituent	a word to which an independent meaning can be assigned
a content word that can be qualified by a modifier