mistress vs fancy woman

mistress and fancy woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
mistress	Yes	No	No	No
fancy woman	Yes	No	No	No

As nouns, mistress and fancy woman are synonyms defined as:

mistress and fancy woman: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other synonyms of mistress include kept woman.

mistress (noun)	fancy woman (noun)
a woman master who directs the work of others	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man	a woman who engages in sexual intercourse for money
a woman schoolteacher (especially one regarded as strict)