A flywheel can be thought of as a kind of mechanical battery, which stores energy in the form of movement (kinetic energy) rather than in a chemical form as in a traditional, electrical battery.

A flywheel is a device used for storing mechanical energy and for smoothing the power output of an engine, comprising a heavy shaft-mounted wheel. Since energy is stored as kinetic energy of rotation, it is advantageous for the wheel to have as large as possible a moment of inertia, and thus for its mass to be concentrated near the rim. The flywheel was initially developed by James Watt.