1. A woman who has a continuing sexual relationship with a man who is married to someone else.
2. 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" (Jane Austen).
a. A woman who owns or keeps an animal: a cat sitting in its mistress's lap.
b. A woman who owns a slave.
4. A woman with ultimate control over something: the mistress of her own mind.
a. A nation or country that has supremacy over others: Great Britain, once the mistress of the seas.
b. Something personified as female that directs or reigns: "my mistress ... the open road" (Robert Louis Stevenson).
6. A woman who has mastered a skill or branch of learning: a mistress of the culinary art.
7. Mistress. Used formerly as a courtesy title when speaking to or of a woman.
8. Chiefly British A woman schoolteacher.