Synchronous speed (r.p.m.) = 60 x Frequency (Hz) / Number of pole pairs
It is the rotational speed of the magnetic field. In an synchronous motor, the rotor rotates at synchronous speed. However, in an asynchronous machine (most of the electric A.C.motors are asynchronous)
the rotation speed is slightly slower than synchronous speed. The difference between synchronous and asynchronous velocity is called slip
Slip is the percentage difference between the rotor speed and the synchronous speed. An asynchronous motor always need some slip to have some mechanical torque, and when slip is zero, torque is also zero.
So, slip depends on the mechanical load of the motor.