Sinusoidal quantity of a frequency that is an integral submultiple of a fundamental frequency.
Even subharmonics are one-half, one-quarter, and so on. Odd subharmonics are one-third, one-fifth, and so on of the fundamental frequency.
They are sometimes called sub-synchronous components.
In the vibration spectrum of a rotating machine, there will normally be a component at the rotational speed along with several harmonics of turning speed. If there is any play in the machine, the spectrum will usually contain sub harmonics. Harmonics of one-half rotational speed are called half orders.