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.

• (anatomy) A membranous tissue composed of one or more layers of cells which forms the covering of most internal and external surfaces of the body and its organs; internally including the lining of vessels and other small cavities, and externally being the skin.