A propeller shaft connects a propeller to an engine. It is also the British English term for the driveshaft which connects an automobile gearbox to a rear differential. It is commonly referred to as a "prop-shaft".

Where the engine and axles are separated from each other, as on four-wheel-drive and rear-wheel-drive vehicles, it is the propeller shaft that serves to transmit the drive force generated by the engine to the axles. For its usage, the optimal shaft is a short, bar-like product. The longer the bar, the more liable it is to sag, and sagging is further promoted when rotation is applied. Sagging causes vibration and results in an increase in noise, to such an extent that the shaft is likely to break when the critical speed is exceeded. The propeller shaft is naturally designed not to break when used within the service limits expected of use. In addition, it is designed to suppress vibrations arising from a wide range of causes.