spouse vs significant other

spouse and significant other both are nouns.

Word	Noun	Adjective	Verb	Adverb
spouse	Yes	No	No	No
significant other	Yes	No	No	No

As nouns, significant other is a hypernym of spouse; that is, significant other is a word with a broader meaning than spouse:

spouse: a person's partner in marriage
significant other: a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship

Other hypernyms of spouse include domestic partner, spousal equivalent, spouse equivalent, relation, relative.

spouse (noun)	significant other (noun)
a person's partner in marriage	a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship