The Collaborative International Dictionary of English v.0.48:

adventitia \adventitia\ n.
   1. the membranous outer covering of an organ or blood vessel.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

adventitia
    n 1: an enveloping or covering membrane or layer of body tissue
         [syn: tunic, tunica, adventitia]