The Collaborative International Dictionary
Plunger \Plun"ger\, n.
One who, or that which, plunges; a diver.
A long solid cylinder, used, instead of a piston or bucket, as a forcer in pumps.
One who bets heavily and recklessly on a race; a reckless speculator. [Cant]
(Pottery) A boiler in which clay is beaten by a wheel to a creamy consistence.
--Knight.-
(Gun.) The firing pin of a breechloader.
Plunger bucket, a piston, without a valve, in a pump.
Plunger pole, the pump rod of a pumping engine.
Plunger pump, a pump, as for water, having a plunger, instead of a piston, to act upon the water. It may be single-acting or double-acting
Wikipedia
A plunger pump is a type of positive displacement pump where the high-pressure seal is stationary and a smooth cylindrical plunger slides through the seal. This makes them different from piston pumps and allows them to be used at higher pressures. This type of pump is often used to transfer municipal and industrial sewage.