Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a permanent magnet machine.
This torque is position dependent and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator.
Cogging torque is an undesirable component for the operation of such a motor.
It is especially prominent at lower speeds, with the symptom of jerkiness.
A slotless and coreless permanent magnet motor does not have any cogging torque.