A massive disc or wheel attached to a rotating shaft to resist changes in rotational speed by its moment of inertia. It absorbs mechanical energy when torque is high and releases it when torque drops. This stabilizes the rotation of internal combustion engines between firing strokes and provides the sudden burst of power needed in punch presses.
Heard on the job
We need to pull the transmission and machine the flywheel; the clutch is chattering badly on engagement.