The Collaborative International Dictionary of English v.0.48:

Water wheel \Wa"ter wheel`\
   1. Any wheel for propelling machinery or for other purposes,
      that is made to rotate by the direct action of water; --
      called an overshot wheel when the water is applied at
      the top, an undershot wheel when at the bottom, a
      breast wheel when at an intermediate point; other forms
      are called reaction wheel, vortex wheel, turbine
      wheel, etc.
      [1913 Webster]

   2. The paddle wheel of a steam vessel.
      [1913 Webster]

   3. A wheel for raising water; a noria, or the like.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water wheel
    n 1: a wheel with buckets attached to its rim; raises water from
         a stream or pond [syn: waterwheel, water wheel]
    2: a wheel that rotates by direct action of water; a simple
       turbine [syn: waterwheel, water wheel]