noun
1. : the main driving wheel or gear of a machine, usually the largest and strongest wheel in the train of mechanism, in some machines (as a shaper) transmitting the power received by the machine to the various mechanisms, in others (as a tractor) delivering to the driving wheels the power transmitted to it from the motor
2. : a drum on which a rope is wound for hauling or lifting (as logs or well-boring tools)
3. : a wheel attached to the boom of a derrick at the base and used to swing the boom about a vertical axis