adventitial

noun Anatomy.

1. the external covering of an organ or other structure, derived from connective tissue, especially the external covering of a blood vessel.

noun

1. the outermost covering of an organ or part, esp the outer coat of a blood vessel

adj.

1. Of or relating to the adventitia of an organ or blood vessel.
2. Adventitious.

n.

1. The outermost membranous covering of an organ or structure, especially the outer coat of an artery.