What is Adventitious?

Legal Definition
Adventitius. From advenio; what comes incidentally; us adventitia bona, goods that, fall to a man otherwise than by inheritance; or adventitia dos, a dowry or portion given by some other friend beside the parent.
-- Bouviers Law Dictionary
Legal Definition
That which comes incidentally, fortuitously or out of the regular course. "Adventitious value" of lands, see Central R. Co., v. State Board of Assessors, 49 N. J. Law, 1, 7 Atl. 306.
-- Black's Law Dictionary