adventitia

ad·ven·ti·tia

noun \ˌad-vən-ˈtish-(ē-)ə, -(ˌ)ven-\

Definition of ADVENTITIA

: the outer layer that makes up a tubular organ or structure and especially a blood vessel, is composed of collagenous and elastic fibers, and is not covered with peritoneum—called also tunica adventitia, tunica externa