1. [n]  a woman master who directs the work of others
2. [n]  an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
3. [n]  a woman schoolteacher

Mistress
Iphiginia Bright poses as the missing Earl of Masters's mistress, hoping to trap her aunt's blackmailer, and is surprised by the sudden appearance of her alleged lover in all his attractiveness and seductiveness. Reprint. LJ.

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1. \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.
            The late queen's gentlewoman! a knight's daughter!
            To be her mistress' mistress!         --Shak.
   2. A woman well skilled in anything, or having the mastery
      over it.
            A letter desires all young wives to make themselves
            mistresses of Wingate's Arithmetic.   --Addison.
   3. A woman regarded with love and devotion; she who has
      command over one's heart; a beloved object; a sweetheart.
      [Poetic] --Clarendon.
   4. A woman filling the place, but without the rights, of a
      wife; a concubine; a loose woman with whom one consorts
      habitually. --Spectator.
   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      woman, married or unmarried, but now superseded by the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.
            Now Mistress Gilpin (careful soul).   --Cowper.
   6. A married woman; a wife. [Scot.]
            Several of the neighboring mistresses had assembled
            to witness the event of this memorable evening.
                                                  --Sir W.
                                                  Scott.
   7. The old name of the jack at bowls. --Beau. & Fl.
   {To be one's own mistress}, to be exempt from control by
      another person.
   

2. \Mis"tress\, v. i.
   To wait upon a mistress; to be courting. [Obs.] --Donne.