The Collaborative International Dictionary of English v.0.48:

Water tower \Wa"ter tow"er\
   1. A large metal pipe made to be extended vertically by
      sections, and used for discharging water upon burning
      buildings.
      [1913 Webster]

   2. A tower or standpipe used as a water storage reservoir, to
      deliver water at a required head of pressure, as to a
      fountain.
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

water tower
    n 1: a large reservoir for water