Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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