An inward stroke, especially a piston stroke moving away from the crankshaft.
As the piston strokes in its bore, the crankshaft rotates, pressurizing the opposite bore reverses rotation.
"Where the entire cycle of events in the cylinder requires four strokes (or two crankshaft revolutions), the engine is called a four-stroke-cycle engine, or a four-cycle engine. The four piston strokes are intake, compression, power and exhaust."