/ˌendəˈTHēlēəm/
Noun, pl. endothelia
1. The thin layer of cells that lines the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. (wikipedia.org)
2. A thin layer of flat epithelial cells that lines the heart, serous cavities, lymph vessels, and blood vessels. (wiktionary.org)
3. The layer of epithelial cells that lines the cavities of the heart and of the blood and lymph vessels and the serous cavities of the body, originating from the mesoderm. (biology-online.org)
4. The tissue that forms a single layer of cells lining various organs and cavities of the body, esp. the blood vessels, heart, and lymphatic vessels. It is formed from the embryonic mesoderm. (Google Dictionary)
5. An epithelium of mesoblastic origin; a thin layer of flattened cells that lines the inside of some body cavities. (wordnetweb.princeton.edu)