endothelium vs epithelium

endothelium and epithelium both are nouns.

Word	Noun	Adjective	Verb	Adverb
endothelium	Yes	No	No	No
epithelium	Yes	No	No	No

As nouns, epithelium is a hypernym of endothelium; that is, epithelium is a word with a broader meaning than endothelium:

endothelium: an epithelium of mesoblastic origin; a thin layer of flattened cells that lines the inside of some body cavities
epithelium: membranous tissue covering internal organs and other internal surfaces of the body

Other hypernyms of endothelium include epithelial tissue.

endothelium (noun)	epithelium (noun)
an epithelium of mesoblastic origin; a thin layer of flattened cells that lines the inside of some body cavities	membranous tissue covering internal organs and other internal surfaces of the body