The Collaborative International Dictionary of English v.0.48:

Entresol \En`tre*sol"\, n. [F.] (Arch.)
   A low story between two higher ones, usually between the
   ground floor and the first story; mezzanine. --Parker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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