domestic partner vs significant other

domestic partner and significant other both are nouns.

Word	Noun	Adjective	Verb	Adverb
domestic partner	Yes	No	No	No
significant other	Yes	No	No	No

As nouns, domestic partner and significant other are synonyms defined as:

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

Other synonyms of domestic partner include spousal equivalent, spouse equivalent.

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