From the archives:
Adventitia comes from the Latin word adventicius, foreign, which in turn is derived from adventus, arrival (formed by ad, to, and venire, to come). The term is used to describe specific layers, typically the outermost layers of certain hollow organs, that develop from nearby tissues.
An example is the tunica adventitia which ostensibly is part of the structure of arteries and veins, forming their outer coats, but in fact is derived from surrounding connective tissue. By the same token, the outer adventitia layer of some alimentary canal organs are also formed from the local connective tissue, in particular those organs, such as the esophagus and rectum, that are not within the peritoneal cavity and thus not in need of a protective serous coating.
Diagram adapted from Wikipedia
Another anatomical use, again in the sense of something "foreign", is seen in the adventitious bursae which form in locations where bursae are usually not found, typically in response to trauma or friction. An example is the bursa that may develop over the ischial tuberosisty in people who sit for extended periods of time. Inflammation of this bursa was called "weaver's bottom" back in the day.
A non-anatomical but related word is advent: coming or arrival.