A woman who has a continuing sexual relationship with a usually married man who is not her husband and from whom she generally receives material support. A woman in a position of authority, control, or ownership, as the head of a household: "Thirteen years had seen her mistress of Kellynch Hall\u201d ( Jane Austen). A woman who owns or keeps an animal: a cat sitting in its mistress's lap. A woman who owns a slave. A woman with ultimate control over something: the mistress of her own mind. A nation or country that has supremacy over others: Great Britain, once the mistress of the seas. Something personified as female that directs or reigns: "my mistress . . . the open road\u201d ( Robert Louis Stevenson). A woman who has mastered a skill or branch of learning: a mistress of the culinary art. Used formerly as a courtesy title when speaking to or of a woman. Chiefly British A woman schoolteacher.