hydraulic ram

Short definition from Wiktionary

Noun

1. 1
   Used other than figuratively or idiomatically: see hydraulic, ram.
2. 2
   A water pump in which a flow of water is intermittently halted by a valve so that the flow is forced upward through an open pipe, usually into a reservoir.
3. 3
   A hydraulic cylinder with a piston activated by pumping hydraulic fluid into the cylinder.