Source : Webster's Revised Unabridged Dictionary (1913)

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

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.

            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      woman, married or unmarried, but now superseded by the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.

Source : WordNet®

mistress
     n 1: an adulterous woman; a woman who has an ongoing extramarital
          sexual relationship with a man [syn: {kept woman}, {fancy
          woman}]
     2: a woman schoolteacher (especially one regarded as strict)
        [syn: {schoolmarm}, {schoolma'am}, {schoolmistress}]
     3: a woman master who directs the work of others