From The Collaborative International Dictionary of English v.0.48 :
Rhododendron \Rho`do*den"dron\, n. [L., fr. Gr. "rodo`dendron,
literally, rose tree; "ro`don rose + de`ndron tree. See
Rose.] (Bot.)
A genus of shrubs or small trees, often having handsome
evergreen leaves, and remarkable for the beauty of their
flowers; rosebay.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
rhododendron
n 1: any shrub of the genus Rhododendron: evergreen shrubs or
small shrubby trees having leathery leaves and showy
clusters of campanulate (bell-shaped) flowers