The Collaborative International Dictionary of English v.0.48:

Epithelium \Ep`i*the"li*um\, n.; pl. E. Epitheliums, L.
   Epithelia. [NL., fr. Gr. 'epi` upon + ? nipple.] (Anat.)
   The superficial layer of cells lining the alimentary canal
   and all its appendages, all glands and their ducts, blood
   vessels and lymphatics, serous cavities, etc. It often
   includes the epidermis (i. e., keratin-producing epithelial
   cells), and it is sometimes restricted to the alimentary
   canal, the glands and their appendages, -- the term
   endothelium being applied to the lining membrane of the blood
   vessels, lymphatics, and serous cavities.
   [1913 Webster]

WordNet (r) 3.0 (2006):

epithelium
    n 1: membranous tissue covering internal organs and other
         internal surfaces of the body [syn: epithelium,
         epithelial tissue]