What is referred to as one complete movement of the piston up or down?

The term that refers to one complete movement of the piston up or down is known as a stroke. In the context of internal combustion engines, each stroke corresponds to a distinct action of the piston within the cylinder. Specifically, this movement can involve the piston moving from the bottom of the cylinder (Bottom Dead Center or BDC) to the top (Top Dead Center or TDC) or vice versa.

Understanding strokes is crucial, as they are the fundamental actions that contribute to the engine's operation, influencing processes like intake, compression, power generation, and exhaust. Each cylinder in a multi-cylinder engine follows this stroke pattern in a coordinated manner to ensure smooth operation.

The other terms, while related to engine functions, refer to different concepts. Compression refers specifically to the phase where the air-fuel mixture is compressed in the cylinder before ignition. Cycle denotes the full sequence of events that includes multiple strokes, such as the four-stroke engine cycle which consists of intake, compression, power, and exhaust strokes. Revolution typically refers to a full rotation of the crankshaft, which may encompass multiple strokes. Hence, "stroke" precisely captures the specific action of the piston moving in one direction.