The Collaborative International Dictionary of English v.0.48:

Avalanche \Av"a*lanche`\ (?; 277), n. [F. avalanche, fr. avaler
   to descend, to let down, from aval down, downward; ? (L. ad)
   + val, L. vallis, valley. See Valley.]
   1. A large mass or body of snow and ice sliding swiftly down
      a mountain side, or falling down a precipice.
      [1913 Webster]

   2. A fall of earth, rocks, etc., similar to that of an
      avalanche of snow or ice.
      [1913 Webster]

   3. A sudden, great, or irresistible descent or influx of
      anything.
      [1913 Webster]

WordNet (r) 3.0 (2006):

avalanche
    n 1: a slide of large masses of snow and ice and mud down a
         mountain
    2: a sudden appearance of an overwhelming number of things; "the
       program brought an avalanche of mail"
    v 1: gather into a huge mass and roll down a mountain, of snow
         [syn: avalanche, roll down]