ad·ven·ti·tious [àdv?n tísh?ss] adjective 1. from outside: added from an outside and often unexpected source rather than intrinsic 2. unusually positioned: developing in an unusual position, as does a root growing downward from a branch [Early 17th century. < medieval Latin adventitius "coming from outside," alteration of Latin adventicius < adventus (see Advent)] -ad·ven·ti·tious·ly, adverb