The Collaborative International Dictionary of English v.0.48:

Mezzanine \Mez"za*nine\, n. [F. mezzanine, It. mezzanino, fr.
   mezzano middle, fr. mezzo middle, half. See Mezzo.]
   1. (Arch.)
      (a) Same as Entresol.
      (b) A partial story which is not on the same level with
          the story of the main part of the edifice, as of a
          back building, where the floors are on a level with
          landings of the staircase of the main house.
          [1913 Webster]

   2. A flooring laid over a floor to bring it up to some height
      or level.
      [Webster 1913 Suppl.]

   3. Also

   mezzanine floor. (Theat.) A floor under the stage, from
      which various contrivances, as traps, are worked.
      [Webster 1913 Suppl.]

   4. The lowest balcony in a theater, or the forward part of
      the first balcony.
      [PJC]

WordNet (r) 3.0 (2006):

mezzanine floor
    n 1: intermediate floor just above the ground floor [syn:
         mezzanine, mezzanine floor, entresol]