- 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(in the past) the female head of a house, especially one who employed servants the mistress of the house Lizzie, fetch your mistress her shawl.
- 3(formal) (old-fashioned) the female owner of a dog or other animal
- 4(of something) (formal) a woman who is in a position of authority or control, or who is highly skilled in something She wants to be mistress of her own affairs (= to organize her own life). She is the mistress of plain speech. compare master