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.
Epithelial cells help to protect or enclose organs. Most produce mucus or other secretions.
Certain types of epithelial cells have tiny hairs called cilia, which help remove foreign substances.
Epithelial cells are arranged in single, or multiple layers, depending on the organ and location.
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.