employer vs mistress

employer and mistress both are nouns.

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

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

Other hyponyms of employer include boss, hirer, master, padrone, Simon Legree, slave driver.

employer (noun)	mistress (noun)
a person or firm that employs workers	a woman master who directs the work of others
	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
	a woman schoolteacher (especially one regarded as strict)