Epithelium (noun)

1. A type of tissue that lines the internal and external surfaces of organs and structures in the body.
2. The layer of cells that covers or lines the internal or external surfaces of an organ or structure in the body.

Origin:

From new latin epithelium, from ancient greek ἐπί (epí, "on") + θήλεια (thēleia, "teat"), from θηλή (thēlē, "nipple, teat").

Examples:

1. The epithelium of the bladder helps prevent urine from being contaminated by bacteria.
2. The epithelium of the skin acts as a barrier against the environment.
3. The epithelium of the small intestine absorbs nutrients from food into the bloodstream.
4. The epithelium of the airways helps to keep the lungs moist and prevent infections.
5. The epithelium of the cornea provides a smooth surface for clear vision.