hydraulic ram

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. The large output piston of a hydraulic press.

GNU Webster's 1913

1. adj. a machine for raising water by means of the energy of the moving water of which a portion is to be raised. When the rush of water through the main pipe d shuts the valve at a, the momentum of the current thus suddenly checked forces part of it into the air chamber b, and up the pipe c, its return being prevented by a valve at the entrance to the air chamber, while the dropping of the valve a by its own weight allows another rush through the main pipe, and so on alternately.

WordNet 3.0

1. n. 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