The Collaborative International Dictionary of English v.0.48:

Weeping willow \Weep"ing wil"low\ (Bot.)
   a tree (Salix babylonica) of the willow family with slender
   leaves, native to China, whose branches grow very long and
   slender, and hang down almost perpendicularly. It grows best
   where soil is moist, as by the banks of streams and is widely
   cultivated as an ornamental tree.

   Syn: Babylonian weeping willow. [PJC]

WordNet (r) 3.0 (2006):

Salix babylonica
    n 1: willow with long drooping branches and slender leaves
         native to China; widely cultivated as an ornamental [syn:
         weeping willow, Babylonian weeping willow, Salix
         babylonica]