The Collaborative International Dictionary of English v.0.48:

Demimonde \Dem`i*monde"\, n. [F.; demi + monde world, L.
   mundus.]
   Persons of doubtful reputation; esp., women who are kept as
   mistresses, though not public prostitutes; demireps.
   [1913 Webster]

   Literary demimonde, writers of the lowest kind.
      [1913 Webster]

WordNet (r) 3.0 (2006):

demimonde
    n 1: a class of woman not considered respectable because of
         indiscreet or promiscuous behavior