mistress vs lover

mistress and lover both are nouns.

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

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

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

Other hypernyms of mistress include adult female, woman.

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