The Collaborative International Dictionary of English v.0.48:

Courtesan \Cour"te*san\ (k?r"t?-z?n; 277), n. [F. courtisane,
   fr. courtisan courtier, It. cortigiano; or directly fr. It.
   cortigiana, or Sp. cortesana. See Court.]
   A woman who prostitutes herself for hire; a prostitute; a
   harlot.
   [1913 Webster]

         Lasciviously decked like a courtesan.    --Sir H.
                                                  Wotton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

courtesan
    n 1: a woman who cohabits with an important man [syn:
         concubine, courtesan, doxy, paramour]