A cylindrically-shaped metal piece (like a plug) that is moved back and forth in a cylinder by pressure from an explosion, or the introduction of liquid or air. In an engine, an explosion in a cylinder head pushes a piston which in turn moves a crank shaft and turns the car's wheels. In a hydraulic system, like your brakes, when the brake pedal is depressed, liquid is pushed against a piston which presses a pad against a rotor, attached to the wheel slowing the car.