dash-pot

n. (Mach.) a mechanical damping device containing a piston that moves in a fluid-filled chamber to serve as a pneumatic or hydraulic cushion for a falling weight, as in the valve gear of a steam engine, to prevent shock. [ 1913 Webster + WordNet 1.5 ]

☞ It consists of a chamber, containing air or a liquid, in which a piston (a), attached to the weight, falls freely until it enters a space (as below the openings, b) from which the air or liquid can escape but slowly (as through cock c), when its fall is gradually checked. [ 1913 Webster ]

A cataract of an engine is sometimes called a dashpot. [ 1913 Webster ]