A driveshaft or driving shaft is a mechanical device for transferring power from the engine or motor to the point where useful work is applied.
Most engines or motors deliver power as rotary motion: this is extracted from the linear motion of pistons in a reciprocating engine; water driving a water wheel; or forced air or water in a turbine. From the point of delivery, the components of power transmission form the drive train. Driveshafts are the rigid rods that transmit the power, typically terminating in gears or universal joints, although belt drives were an early transfer mechanism.
Driveshafts are carriers of torque: they are subject to torsion and shear stress, which represents the difference between the input force and the load. They thus need to be strong enough to bear the stress, without imposing too great an additional inertia by virtue of the weight of the shaft.