[ep.i.the.li.um] n, pl -lia [NL, fr. epi- + Gk thele nipple--more at feminine] (1748) 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