mis ‧ tress /ˈmɪstrəs, ˈmɪstrɪs/ BrE AmE noun [countable]
[ Date: 1200-1300 ; Language: Old French ; Origin: maistresse , from maistre ; ⇨ ↑ master 1 ]
1 . 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-fashioned a female teacher ⇨ master :
the new English mistress
3 . the female owner of a dog, horse etc ⇨ master
4 . old-fashioned the female employer of a servant ⇨ master :
The maid looked nervously at her mistress.
5 . be (a/the) mistress of something 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 . Mistress old use used with a woman’s family name as a polite way of speaking to her ⇨ master
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THESAURUS
▪ girlfriend a girl or woman that you have a romantic relationship with, especially for a fairly long time:
Katherine was his first girlfriend.
▪ partner the person you are married to, or the person you are living with and having a sexual relationship with:
He brought his partner along to the party.
▪ old flame informal someone who was your girlfriend or boyfriend in the past:
I found a love letter from one of his old flames.
▪ mistress a woman that a man has a sexual relationship with even though he is married to someone else. Often used when the man is famous or powerful:
The president had a mistress for many years.
▪ lover someone who you have a sexual relationship with, but who you are not married to:
He was going to meet his secret lover.
▪ fiancée the woman who a man is going to marry:
Rod plans to marry his fiancée later this year.
▪ ex informal a man's former wife or girlfriend:
My ex looks after the children on Thursdays.
▪ be going out with somebody if you are going out with a girl or woman, she is your girlfriend:
She's going out with a guy she met at work.