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Webster 1913 Edition

Adj.

Accidental.

(Bot.)

Adventitious.

Gray.

Noun.

A thing or person coming from without; an immigrant.

[R.]

Bacon.

Adj.

Accidental; adventitious. [Little used.]

adventive ‎(comparative more adventive, superlative most adventive)

accidental
adventitious
(biology) Of a plant that is not native, but was introduced by humans to a place and has since become naturalized.

adventive ‎(plural adventives)

A non-native plant that has become naturalized.
- 1988 April 15, James Krohe Jr., “Where Has All the Flora Gone?”, in Chicago Reader:
  Such interlopers are known as exotics, adventives, or aliens, all terms that may be considered synonymous with "nasty.

adventive