a woman in a position of power, authority, possession, or control
archaic the female head of a household
a woman who has achieved mastery of a subject or skill
a woman with whom a man has a continuing sexual relationship outside marriage
chiefly Brit a female schoolteacher
archaic a sweetheart
(Mistress) archaic used as a title preceding the name of a woman and now superseded by Mrs, Miss, and Ms
[Middle English maistresse, from Old French maistresse, fem of maistre master]