Source : Webster's Revised Unabridged Dictionary (1913)

Endothelium \En`do*the"li*um\, n.; pl. {Endothelia}. [NL., fr.
   Gr. 'e`ndon within + ? nipple.] (Anat.)
   The thin epithelium lining the blood vessels, lymphatics, and
   serous cavities. See {Epithelium}.

Source : WordNet®

endothelium
     n : an epithelium of mesoblastic origin; a thin layer of
         flattened cells that lines the inside of some body
         cavities
     [also: {endothelia} (pl)]