A flywheel is defined by the sci-tech encyclopedia as a rotating mass used to maintain the speed of a machine between given limits while the machine receives or releases energy at a varying rate.
It is a device which has a high moment of inertia and stores rotational energy also known as kinetic energy. The flywheel stores energy on increase in its speed and releases energy on decrease in its speed by using a rotor and a generator. This process is also known as charging and discharging. During discharging, stored kinetic energy is transformed into electrical energy which is delivered constantly using a suitable control system. It thus acts as a storage device for energy.