The Collaborative International Dictionary of English v.0.48:

Water table \Wa"ter ta"ble\
   1. (Arch.) A molding, or other projection, in the wall of a
      building, to throw off the water, -- generally used in the
      United States for the first table above the surface of the
      ground (see Table, n., 9), that is, for the table at the
      top of the foundation and the beginning of the upper wall.
      [1913 Webster]

   2. (Hydraulic Engin.) The upper limit of the portion of the
      ground wholly saturated with water. The water table may be
      within a few inches of the surface or many feet below it.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

water table
    n 1: underground surface below which the ground is wholly
         saturated with water; "spring rains had raised the water
         table" [syn: water table, water level, groundwater
         level]