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.
Synonym: tunica adventitia, membrana adventitia.
Origin: L. Adventicius, coming from abroad, foreign, fr. Ad, to + venio, to come
(05 Mar 2000)