n. a heavy wheel attached to machinery to equalize the movement (opposing any sudden acceleration by its inertia and any retardation by its momentum), and to accumulate or give out energy for a variable or intermitting resistance. See Fly, n., 9.n. A heavy wheel or disk which stores kinetic energy by rotating on a shaft, and by its momentum smoothes the operation of a reciprocating engine by reducing fluctuations of speed. It is used in certain types of machinery, such as automobiles.