What is meant by a critical speed for rotating machinery?

The critical speed of a shaft is the speed at which it will shear due to excessive torque placed upon it by the prime mover in driving the load.

The critical speed of a shaft is potentially damaging radial vibration that exists when rotational frequencies coincide with one or more of the shaft system natural frequencies.

The critical speed of a shaft is the speed at which it will fly apart due to excessive centrifugal forces associated with excessive speed.

The critical speed of a shaft is the speed of rotation that is out of harmonic synchronism with the shaft's natural vibration frequency which produces excessive vibration.

What is meant by a critical speed for...