The Collaborative International Dictionary of English v.0.48:

Mesothelium \Mes`o*the"li*um\, n. [NL., fr. Gr. me`sos middle +
   E. epithelium.] (Biol.)
   Epithelial mesoderm; a layer of cuboidal epithelium cells,
   formed from a portion of the mesoderm during the
   differetiation of the germ layers. It constitutes the
   boundary of the c[oe]lum.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mesothelium
    n 1: epithelium originating in the embryonic mesoderm; lines the
         primordial body cavity