Mistress is an old-fashioned term for the lady of the house, especially one who is head of the household.
A mistress (in full, schoolmistress) also a female schoolteacher. The term is now generally obsolete (even in Britishpublic schools). The term 'Headmistress' also survives in some American and Commonwealth private schools. See schoolmaster.
Mistress is also an old form of address for a woman, for example Mistress Quickly in Shakespeare's The Merry Wives of Windsor. The titleMistress 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.
One's mistress is one's female lover, especially if one is married or in an otherwise 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.