water tower

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A standpipe or elevated tank used as a reservoir or for maintaining equal pressure in a water system.
• noun A firefighting apparatus for lifting hoses to the upper levels of a tall structure.

from The Century Dictionary.

• noun Same as stand-pipe, 7.
• noun A tower, in a system of water-supply, in which water is kept to maintain a constant head or pressure and also to permit a considerable draft of water to be made from the system, in case of fire, before the pumping-engines are speeded up to the increased demand.
• noun A tower in which a spray of falling water cools and cleanses an ascending current of gas.

from the GNU version of the Collaborative International Dictionary of English.

• A large metal pipe made to be extended vertically by sections, and used for discharging water upon burning buildings.
• A tower or standpipe used as a water storage reservoir, to deliver water at a required head of pressure, as to a fountain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A large tank of water, in an elevated position, acting as a local reservoir; water is pumped into the tank and fed by gravity to the consumers.
• noun archaic A large metal pipe made to be extended vertically by sections, and used for discharging water upon burning buildings.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a large reservoir for water