n. 1 a female head of a household. 2 a a woman in authority over others. b the female owner of a pet. 3 a woman with power to control etc. (often foll. by of: mistress of the situation). 4 Brit. a a female teacher (music mistress). b a female head of a college etc. 5 a a woman (other than his wife) with whom a married man has a (usu. prolonged) sexual relationship. b archaic or poet. a woman loved and courted by a man. 6 archaic or dial. (as a title) = MRS. Mistress of the Robes a lady in charge of the Queen's wardrobe.
[ ME f. OF maistresse f. maistre MASTER ]