1. [n]  a woman who cohabits with an important man
2. [n]  a woman's lover

Paramour
Paramour

more details ...

1. \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.
            The seducer appeared with dauntless front,
            accompanied by his paramour           --Macaulay.
   2. Love; gallantry. [Obs.] ``For paramour and jollity.''
      --Chaucer.
   

2. \Par"a*mour`\, Paramours \Par"a*mours`\, adv.
   By or with love, esp. the love of the sexes; -- sometimes
   written as two words. [Obs.]
         For par amour, I loved her first ere thou. --Chaucer.