flywheel
n.a heavy wheel that spins inside a machine to keep it moving smoothly. It stores energy so the machine doesn't stop when the power goes out.
n.a mechanical device used to store rotational energy and provide a smooth, continuous output of power. It consists of a heavy wheel with a large moment of inertia, designed to maintain momentum during fluctuations in the power source.
“The flywheel helps the engine run more smoothly.”
“In a steam engine, the flywheel ensures that the pistons continue moving even when the pressure drops between cycles.”
“Modern hybrid vehicles often use a flywheel to capture kinetic energy during braking, which is then released to assist the engine during acceleration, improving overall fuel efficiency.”
From fly + wheel.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.