lover vs significant other

lover and significant other both are nouns.

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

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

lover: a significant other to whom you are not related by marriage
significant other: a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship

Other hypernyms of lover include domestic partner, spousal equivalent, spouse equivalent.

lover (noun)	significant other (noun)
a person who loves someone or is loved by someone	a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship
an ardent follower and admirer
a significant other to whom you are not related by marriage