Driveshaft
Most engines or motors create power as torque by making something rotate.  This can be from an internal combustion engine (as in a car), water driving a water wheel, or gas or water going through a turbine.  This rotating power is called torque.  The load that is turned creates torsion and shear stress.  Drive shafts need to be strong enough to take this stress.  A light drive shaft will more efficiently move the power to the load, so there needs to be a balance between strength and weight.