/ ˈmɪstrəs; NAmE / noun
1.
a man's (usually a married man's) mistress is a woman that he is having a regular sexual relationship with and who is not his wife
2.
( BrE , old-fashioned ) a female teacher in a school, especially a private school :
the Biology mistress
3.
(in the past) the female head of a house, especially one who employed servants :
the mistress of the house
4.
the female owner of a dog or other animal
5.
( formal ) a woman who is in a position of authority or control, or who is highly skilled in sth :
She wants to be mistress of her own affairs (= to organize her own life) .
—compare master
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WORD ORIGIN
Middle English : from Old French maistresse , from maistre master.