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]

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