The term mistress is the feminine form of the word master. It has several related meanings:

• A mistress is an old-fashioned term for the lady of the house, especially one who is head of the household.
• A mistress is the female lover of a person, usually one who is married or in a committed relationship with another. In some systems of nobility (especially in the case of royalty), mistresses had official titles and positions at court. See intimate relationship.
• Mistress is also an old form of address for a woman, for example Mistress Quickly in Shakespeare's The Merry Wives of Windsor. The title Mistress did not distinguish between married and unmarried women. The title Mrs. is an abbreviation of Mistress, and the titles Miss and Ms. are also derived from it.
• In BDSM, mistress is another term for dominatrix.
• A mistress (in full, schoolmistress) was also a female school teacher. The term is now generally obsolete except in British public schools. The term 'Headmistress' also survives in some American and Commonwealth private schools. See schoolmaster.