The Collaborative International Dictionary of English v.0.48:

Mistress \Mis"tress\, n. [OE. maistress, OF. maistresse, F.
   ma[^i]tresse, LL. magistrissa, for L. magistra, fem. of
   magister. See Master, Mister, and cf. Miss a young
   woman.]
   1. A woman having power, authority, or ownership; a woman who
      exercises authority, is chief, etc.; the female head of a
      family, a school, etc.
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            The late queen's gentlewoman! a knight's daughter!
            To be her mistress' mistress!         --Shak.
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   2. A woman well skilled in anything, or having the mastery
      over it.
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            A letter desires all young wives to make themselves
            mistresses of Wingate's Arithmetic.   --Addison.
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   3. A woman regarded with love and devotion; she who has
      command over one's heart; a beloved object; a sweetheart.
      [Poetic] --Clarendon.
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   4. A woman filling the place, but without the rights, of a
      wife; a woman having an ongoing usually exclusive sexual
      relationship with a man, who may provide her with
      financial support in return; a concubine; a loose woman
      with whom one consorts habitually; as, both his wife and
      his mistress attended his funeral. --Spectator.
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   5. A title of courtesy formerly prefixed to the name of a
      woman, married or unmarried, but now superseded by the
      contracted forms, Mrs., for a married, and Miss, for an
      unmarried, woman.
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            Now Mistress Gilpin (careful soul).   --Cowper.
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   6. A married woman; a wife. [Scot.]
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            Several of the neighboring mistresses had assembled
            to witness the event of this memorable evening.
                                                  --Sir W.
                                                  Scott.
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   7. The old name of the jack at bowls. --Beau. & Fl.
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   To be one's own mistress, to be exempt from control by
      another person.
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