Source : Webster's Revised Unabridged Dictionary (1913)

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

Water tower \Water tower\
   A tower or standpipe used as a reservoir to deliver water at
   a required head, as to a fountain.

Source : WordNet®

water tower
     n : a large reservoir for water