lover vs mistress

lover and mistress both are nouns.

Word	Noun	Adjective	Verb	Adverb
lover	Yes	No	No	No
mistress	Yes	No	No	No

As nouns, mistress is a hyponym of lover; that is, mistress is a word with a more specific, narrower meaning than lover:

lover: a significant other to whom you are not related by marriage
mistress: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other hyponyms of lover include fancy man, paramour, man, fancy woman, kept woman.

lover (noun)	mistress (noun)
a person who loves someone or is loved by someone	a woman master who directs the work of others
an ardent follower and admirer	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
a significant other to whom you are not related by marriage	a woman schoolteacher (especially one regarded as strict)