The Collaborative International Dictionary of English v.0.48:

Waterfall \Wa"ter*fall`\, n.
   1. A fall, or perpendicular descent, of the water of a river
      or stream, or a descent nearly perpendicular; a cascade; a
      cataract.
      [1913 Webster]

   2. (Hairdressing) An arrangement of a woman's back hair over
      a cushion or frame in some resemblance to a waterfall.
      [1913 Webster]

   3. A certain kind of neck scarf. --T. Hughes.
      [1913 Webster] Water feather

WordNet (r) 3.0 (2006):

waterfall
    n 1: a steep descent of the water of a river [syn: waterfall,
         falls]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "waterfall":
   Niagara, cascade, cataract, chute, collapse, comedown, crash,
   debacle, declension, declination, defluxion, descending,
   descension, descent, down, downbend, downcome, downcurve, downfall,
   downflow, downgrade, downpour, downrush, downtrend, downturn,
   downward trend, drop, dropping, eddy, fall, falling, falls, force,
   gravitation, inclination, linn, nappe, plummeting, pounce, rapids,
   riffle, sault, shoot, spout, stoop, surge, swoop, vortex,
   watershoot, whirlpool