1. Referring to an addition from without; extrinsically added: The adventitious population in the big city is due to people coming from many different countries from around the world, while the minority of the population was born there.
2. Regarding something that appears sporadically, or out of the normal place: Jane noticed in her garden that some of the flowers she had planted in the fall were evidently quite adventitious and were suddenly appearing in a completely different spot!
3. Not natural or hereditary; pertaining to a growth in an unexpected place on a plant: Examples of such adventitious developments on organisms are roots that form on stems, an increase of hair where it usually is not found, or the development of a plant in a foreign habitat.

Adventitious roots, shoots, buds, etc. are produced in unusual parts of the plant.