/ˌadvenˈtiSH(ē)ə/
Noun
1. The outermost connective tissue covering of any organ, vessel, or other structure.(wikipedia.org)
2. The outermost layer of the wall of a blood vessel. (Google Dictionary)
3. The external covering of an organ or other structure, derived from connective tissue, especially the external covering of a blood vessel. (dictionary.reference.com)
4. The outermost connective tissue covering of any organ, vessel, or other structure not covered by a serosa; instead, the covering is properly derived from without (i.e., from the surrounding connective tissue) and does not form an integral part of such organ or structure. (biology-online.org)
Adjective: adventitial
Synonym: tunica adventitia, membrana adventitia
Word origin: Latin adventīcia, neuter plural of adventīcius adventitious