(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.

plural of epithelium

Any of a series of secretive layers of cells found in the uterus, parts of the anus and eye, the male urethra and vas deferens and in parts of the pharynx.

The black colored layer of the iris, located behind the iris.