piston (reciprocating engine component). A movable plug inside the cylinder of a reciprocating engine that moves up and down to compress the fuel-air mixture and to transmit the force from the expanding gases in the cylinder to the crankshaft. The piston is sealed in the cylinder by cast-iron piston rings and connected to the crankshaft through a connecting rod.
piston displacement (reciprocating engine specification). The total volume, in cubic inches, cubic centimeters, or liters swept by all of the pistons of a reciprocating engine as they move in one revolution of the crankshaft.
piston engine (heat engine type). One of the common terms used for a reciprocating engine.
piston pin (reciprocating engine component). The hardened and polished steel pin used to connect the piston of a reciprocating engine to the connecting rod. Piston pins are also called wrist pins and gudgeon pins.
piston rings (reciprocating engine component). Metal rings, usually made of cast iron, that fit into grooves around the outside of a reciprocating-engine piston. Piston rings form a seal between the piston and the cylinder wall to prevent the high-pressure air in the cylinder from leaking out.

piston-type pump (fluid pump). A mechanical pump which moves fluid by pistons moving back and forth in the pump cylinders. In an axial-piston pump, the cylinders are arranged parallel to the pump axis, and in a radial-piston pump, the cylinders radiate out perpendicular to the axis.
pit (corrosion). A pocket of chemical salts that forms on the surface of a metal when it corrodes.
pitch (aircraft maneuver). Rotation of an aircraft about its lateral axis.
pitch (rivet layout dimension). The distance between the centers of adjacent rivets installed in the same row.

pitch (thread dimension). The linear distance, measured parallel to the length of a threaded fastener, between corresponding points on two adjacent threads.
pitch angle (helicopter rotor blade). The angle between the chord line of a rotor blade and the reference plane of the main rotor hub, or the plane of rotation of the rotor.
pitch angle (propeller specification). The angle between the chord line of a propeller blade and the plane of rotation.
pitch axis (aircraft axis). The lateral axis of an aircraft that extends from wing tip to wing tip and passes through the center of gravity. This is the axis about which the aircraft pitches. See axes of an aircraft.