Definition of Endothelial

Endothelial: Relating to the endothelium, the layer of flat cells lining the closed spaces of the body such as the inside of blood vessels, lymphatic vessels, the heart, and body cavities.

By contrast, the outside layer of cells that covers all the free, open surfaces of the body including the skin and mucous membranes that communicate with the outside of the body is called the epithelium.

The word "endothelium" is derived from the Greek "endon," within + G. "thele," nipple, so it literally means within the nipple.