1. The outermost connective tissue covering of any organ, vessel, or other structure.(wikipedia.org)
2. The outermost layer of epithelial tissue encasing a visceral organ. (wiktionary.org)
3. 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)
4. An enveloping or covering membrane or layer of body tissue. (wordnetweb.princeton.edu)
Syn: tunica adventitia, membrana adventitia, tunica externa.
Word origin: From Latin adventicius, coming from abroad, foreign, fr. Ad, to _ venio, to come