Source : Webster's Revised Unabridged Dictionary (1913)

Adventive \Ad*ven"tive\, a.
   1. Accidental.

   2. (Bot.) Adventitious. --Gray.

Adventive \Ad*ven"tive\, n.
   A thing or person coming from without; an immigrant. [R.]
   --Bacon.

Source : WordNet®

adventive
     adj : not native and not fully established; locally or temporarily
           naturalized; "an adventive weed"