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.
      [1913 Webster]

            The late queen's gentlewoman! a knight's daughter!
            To be her mistress' mistress!         --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster +PJC]

   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      woman, married or unmarried, but now superseded by the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.
      [1913 Webster]

            Now Mistress Gilpin (careful soul).   --Cowper.
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   6. A married woman; a wife. [Scot.]
      [1913 Webster]

            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.
      [1913 Webster]

   To be one's own mistress, to be exempt from control by
      another person.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mistress \Mis"tress\, v. i.
   To wait upon a mistress; to be courting. [Obs.] --Donne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "Mistress":
   Frau, Fraulein, Miss, Mlle, Mme, Mmes, dame, dona, donna, lady,
   madam, madame, mademoiselle, mem-sahib, mesdames, senhora,
   senhorita, signora, signorina, vrouw



Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "mistress":
   Dulcinea, abbess, beneficiary, best girl, cestui, cestui que trust,
   cestui que use, chatelaine, concubine, dame, deedholder, dowager,
   doxy, dream girl, duenna, educatress, feoffee, feudatory,
   first lady, gill, girl, girl friend, goodwife, governess,
   great lady, headmistress, homemaker, householder, housewife,
   inamorata, instructress, jill, jo, kept mistress, kept woman, lady,
   lady love, laird, landlady, landlord, lass, lassie, lord, lover,
   madam, master, matriarch, matron, mesne, mesne lord,
   mother superior, odalisque, old lady, owner, paramour, playmate,
   proprietary, proprietor, proprietress, proprietrix, rentier,
   schooldame, schoolmarm, schoolmistress, squire, titleholder,
   tutoress, unofficial wife, woman