General:
* Rotator: A generic term for something that rotates.
* Inertia wheel: Emphasizes the flywheel's role in storing kinetic energy.
* Energy storage device: A more technical term for its function.
* Momentum wheel: Highlights the flywheel's ability to store angular momentum.
Specific uses:
* Crankshaft: In engines, the flywheel is attached to the crankshaft.
* Rotor: In generators and motors, the flywheel is part of the rotating component.
* Momentum disc: Used in spacecraft for attitude control.
Informal:
* Spinning wheel: A simple, descriptive term.
* Heavy wheel: Emphasizes the flywheel's weight.
Ultimately, the best word to use depends on the specific situation and the desired level of formality.