A flywheel is a heavy shaft-mounted rotating disc that absorbs and stores twisting or spinning motion and then releases it as rotational kinetic energy to provide motion to a stationary, or nearly stationary object.
Flybrids (a variation of regular electromechanical hybrids) use a flywheel instead of a battery to store regenerative braking energy. This stored energy is used to initially propel (or assist the vehicles internal combustion engine) for powering and maintaining motion of the vehicle.

