plunger

A plunger is a component used in various mechanical devices, most notably in pumps. In the context of pumps, a plunger is a type of piston that moves back and forth within a cylinder. Unlike some other piston types, a plunger pump's plunger typically moves through a stationary packed seal.1

Plunger pumps are a form of positive displacement pump, meaning they move a specific volume of fluid with each cycle. They are among the oldest types of pumps still in common use and are known for their reliability and durability. A plunger pump can be described as a force pump. A plunger bucket is specifically a piston without a valve used in a pump.123