Adventitia

n. outer covering of an organ or blood vessel

For the botany term, see adventitious. Adventitia is the outermost connective tissue covering of any organ, vessel, or other structure. For example, the connective tissue that surrounds an artery is called the tunica adventitia because it is considered extraneous to the artery.To some degree, its role is complimentary to that of the serosa, which also provides a layer of tissue surrounding an organ. In the abdomen, whether an organ is covered in adventitia or serosa depends upon whether it is peritoneal or retroperitoneal:peritoneal organs are covered in serosa (a layer of mesothelium, the visceral peritoneum) retroperitoneal organs are covered in adventitia (loose connective tissue)
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Adventitia /ad·ven·ti·tia/ 1. adventitial. 2. tunica adventitia. ad·ven·ti·tia n. The outermost membranous covering of an organ or structure, [more]Adventitia - Community and Resources

Noun
1. an enveloping or covering membrane or layer of body tissue
(synonym) tunic, tunica
(hypernym) membrane, tissue layer
(hyponym) albuginea

noun
(tunica) adventitia = outer layer of the wall of an artery or vein