hydraulic ram

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

• noun A water pump in which the downward flow of naturally running water is intermittently halted by a valve so that the flow is forced upward through an open pipe into a reservoir.
• noun The large output piston of a hydraulic press.

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

• noun a water pump that uses the kinetic energy of flowing water to force a small fraction of that water to a reservoir at a higher level