The Collaborative International Dictionary of English v.0.48:

R'egime \R['e]`gime"\ (r?`zh?m"), n. [F. See Regimen.]
   1. Mode or system of rule or management; character of
      government, or of the prevailing social system.
      [1913 Webster]

            I dream . . . of the new r['e]gime which is to come.
                                                  --H. Kingsley.
      [1913 Webster]

   2. (Hydraul.) The condition of a river with respect to the
      rate of its flow, as measured by the volume of water
      passing different cross sections in a given time, uniform
      r['e]gime being the condition when the flow is equal and
      uniform at all the cross sections.
      [1913 Webster]

   The ancient r['e]gime, or Ancien r['e]gime [F.], the
      former political and social system, as distinguished from
      the modern; especially, the political and social system
      existing in France before the Revolution of 1789.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ancien regime
    n 1: a political and social system that no longer governs
         (especially the system that existed in France before the
         French Revolution)