|Origin:||maistresse, from maistre; MASTER1|
a woman that a man has a sexual relationship with, even though he is married to someone else:
The Prince had shocked society by living openly with his mistress.
2 British English old-fashionedSES
a female teacher [↪ master]:
the new English mistress
the female owner of a dog, horse etc [↪ master]
the female employer of a servant [↪ master]:
The maid looked nervously at her mistress.
if a woman is a mistress of something, she is in control of it, highly skilled at it etc [↪ master]:
She appeared to be very much the mistress of the situation.
6 old use
used with a woman's family name as a polite way of speaking to her [↪ master]