Webster's English Dictionary

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epithelium

ep.i.the.li.um \.ep-*-'the--le--*m\ n [NL, fr. epi- + Gk the-le- nipple - 
   more at FEMININE] 1: a membranous cellular tissue that covers a free 
   surface or lines a tube  or cavity of an animal body and serves esp. to 
   enclose and protect the other parts of the body, to produce secretions and 
   excretions, and to function in assimilation 2: a usu. thin layer of 
   parenchyma that lines a cavity or tube of a plant