flywheel
    n 1: regulator consisting of a heavy wheel that stores kinetic
         energy and smooths the operation of a reciprocating engine

flywheel \flywheel\ 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.
   [WordNet 1.5 +PJC]

   Note: Flywheels rotating at high speed have also been
         proposed as a means to store kinetic energy for use as
         a low-polluting source of energy in vehicles.
         [PJC]