Falling or liable to fall, especially after a definite period of time; not perennial or permanent.
Specifically— In botany: Falling off at maturity or at the end of the season, as petals, leaves, fruit, etc.: in distinction from fugacious or caducous organs, which fall soon after their appearance, and from persistent or permanent, or, as applied to leaves, from evergreen. Losing the foliage every year: as, deciduous trees.
In zoology: Falling off at a certain stage of an animal's existence, as the hair, horns, and teeth of certain animals. Losing certain parts regularly and periodically, or at certain stages or ages: as, a deciduous insect.