The Collaborative International Dictionary of English v.0.48:

Paramour \Par"a*mour\, n. [F. par amour, lit., by or with love.
   See 2d Par, and Amour.]
   1. A lover, of either sex; a wooer or a mistress (formerly in
      a good sense, now only in a bad one); one who takes the
      place, without possessing the rights, of a husband or
      wife; -- used of a man or a woman.
      [1913 Webster]

            The seducer appeared with dauntless front,
            accompanied by his paramour           --Macaulay.
      [1913 Webster]

   2. Love; gallantry. [Obs.] "For paramour and jollity."
      --Chaucer.
      [1913 Webster] Paramour

The Collaborative International Dictionary of English v.0.48:

Paramour \Par"a*mour`\, Paramours \Par"a*mours`\, adv.
   By or with love, esp. the love of the sexes; -- sometimes
   written as two words. [Obs.]
   [1913 Webster]

         For par amour, I loved her first ere thou. --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

paramour
    n 1: a woman's lover [syn: fancy man, paramour]
    2: a woman who cohabits with an important man [syn: concubine,
       courtesan, doxy, paramour]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "paramour":
   Casanova, admirer, adorer, amorist, concubine, doxy, follower,
   infatuate, kept mistress, kept woman, lover, mistress, playmate,
   pursuer, suitor, unofficial wife, woman, wooer