NOUN (1)

1. an epithelium of mesoblastic origin; a thin layer of flattened cells that lines the inside of some body cavities;

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]