flywheel

Etymology

fly + wheel

noun

1. (mechanical engineering) A rotating mass used to maintain the speed of a machine within certain limits while the machine receives or releases energy at a varying rate, or as a form of energy storage.
   As on Nos. 20001-3, the motor and generator armature shafts of the new locomotive each carry a heavy flywheel to provide kinetic energy and help maintain the speed of the motor-generator set during interruptions of supply, as at breaks in the continuity of the conductor rail. 1959 March, “The 2,500 h.p. electric locomotives for the Kent Coast electrification”, in Trains Illustrated, page 123