The term "epithelium" refers to layers of cells that line hollow organs and glands. It is also those cells that make up the outer surface of the body.
Kierszenbaum AL, Tres LL. Epithelium. In: Kierszenbaum AL, Tres LL, eds. Histology and Cell Biology: An Introduction to Pathology. 4th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 1.