Adventitia: The outermost connective tissue covering of any organ, vessel, or other structure. For example, the connective tissue that surrounds an artery is called the adventitia because it is considered extraneous to the artery.

"Adventitia" comes from the Latin "adventicius" meaning "foreign, strange, extraneous, coming from abroad or from outside." Other English words that stem from "advenire" include "adventure" and "avenue."

A related term is "adventititious" meaning "coming from an external source or occurring in an unusual place or manner." When a doctor or nurse listens to the chest or abdomen, adventitious sounds are those that are normally not heard coming from the chest or abdomen.