A piston is a disk, or short cylinder, that is a close-fitting component within a tube in which it moves up and down against a liquid or gas. It is used in pumps in trenchless construction to impart motion. A piston is a component of a mud pump used in horizontal directional drilling.
Trenchlesspedia Explains Piston
The mud pump is a reciprocating pump that uses a large-surface piston, in a manner similar to that of an automotive piston, to move drilling fluid downhole and back to the surface under pressures of more than 7,000 psi. Drilling mud washes and secures the surface of the bore, preventing its collapse. Mud pumps, which pump the fluid down the drill string and return it to the surface where it's cleaned and recirculated, use a piston to drive the mud through the process. Because the mud must travel to the bottom of the hole and back up to the rig, great pressures are built up in the drill string and bore. The piston is specially designed to withstand these pressures.