open marriage vs marriage

open marriage and marriage both are nouns.

Word	Noun	Adjective	Verb	Adverb
open marriage	Yes	No	No	No
marriage	Yes	No	No	No

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

open marriage: a marriage in which each partner is free to enter into extraneous sexual relationships without guilt or jealousy from the other
marriage: the state of being a married couple voluntarily joined for life (or until divorce)

Other hypernyms of open marriage include matrimony, spousal relationship, union, wedlock.

open marriage (noun)	marriage (noun)
a marriage in which each partner is free to enter into extraneous sexual relationships without guilt or jealousy from the other	the act of marrying; the nuptial ceremony
	two people who are married to each other
	the state of being a married couple voluntarily joined for life (or until divorce)
	a close and intimate union