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]

The Collaborative International Dictionary of English v.0.48:

Willow \Wil"low\, n. [OE. wilowe, wilwe, AS. wilig, welig; akin
   to OD. wilge, D. wilg, LG. wilge. Cf. Willy.]
   [1913 Webster]
   1. (Bot.) Any tree or shrub of the genus Salix, including
      many species, most of which are characterized often used
      as an emblem of sorrow, desolation, or desertion. "A
      wreath of willow to show my forsaken plight." --Sir W.
      Scott. Hence, a lover forsaken by, or having lost, the
      person beloved, is said to wear the willow.
      [1913 Webster]

            And I must wear the willow garland
            For him that's dead or false to me.   --Campbell.
      [1913 Webster]

   2. (Textile Manuf.) A machine in which cotton or wool is
      opened and cleansed by the action of long spikes
      projecting from a drum which revolves within a box studded
      with similar spikes; -- probably so called from having
      been originally a cylindrical cage made of willow rods,
      though some derive the term from winnow, as denoting the
      winnowing, or cleansing, action of the machine. Called
      also willy, twilly, twilly devil, and devil.
      [1913 Webster]

   Almond willow, Pussy willow, Weeping willow. (Bot.) See
      under Almond, Pussy, and Weeping.

   Willow biter (Zool.) the blue tit. [Prov. Eng.]

   Willow fly (Zool.), a greenish European stone fly
      (Chloroperla viridis); -- called also yellow Sally.

   Willow gall (Zool.), a conical, scaly gall produced on
      willows by the larva of a small dipterous fly (Cecidomyia
      strobiloides).

   Willow grouse (Zool.), the white ptarmigan. See
      ptarmigan.

   Willow lark (Zool.), the sedge warbler. [Prov. Eng.]

   Willow ptarmigan (Zool.)
      (a) The European reed bunting, or black-headed bunting.
          See under Reed.
      (b) A sparrow (Passer salicicolus) native of Asia,
          Africa, and Southern Europe.

   Willow tea, the prepared leaves of a species of willow
      largely grown in the neighborhood of Shanghai, extensively
      used by the poorer classes of Chinese as a substitute for
      tea. --McElrath.

   Willow thrush (Zool.), a variety of the veery, or Wilson's
      thrush. See Veery.

   Willow warbler (Zool.), a very small European warbler
      (Phylloscopus trochilus); -- called also bee bird,
      haybird, golden wren, pettychaps, sweet William,
      Tom Thumb, and willow wren.
      [1913 Webster]

WordNet (r) 3.0 (2006):

weeping willow
    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]