1. regulator consisting of a heavy wheel that stores kinetic energy and smooths the operation of a reciprocating engine
The Collaborative International Dictionary of English v.0.48:
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.