coming accidentally or unexpectedly from another source; extraneous
And if there be any original defects, or adventitious ones introduced by time or corruption, it is not an easy thing to get them changed — John Locke
said of part of a plant or animal: occurring sporadically or in an unusual place
adventitious buds
adventitiously adv
adventitiousness noun
[Latin adventicius coming from outside, from adventus: see adventure1]