Induction motor slip speed stator and slip difference
The synchronous speed ns of the stator field is directly related to the mains frequency and the number of (magnetic) poles in the stator and is expressed in revolutions per minute [rpm]:

$$n_s = \dfrac{120×f}{p} \text{[rpm]}$$

The rotor speed nr is is the number of revolutions per minute of the rotor.

Slip is the ratio of the speed [difference between stator and rotor] to stator field speed. Slip is defined as:

$$s=\dfrac{n_s-n_r}{n_s} $$

So if the rotor is:


at the same speed as the magnetic field then slip is 0;
at a lower speed than the magnetic field then slip is positive (for normal motor mode this is in the order of 5%);
at a higher speed than the magnetic field then the slip is negative. The motor is in generator mode.


Li-aung Yip explained in his comment that slip speed is probably defined by the difference in stator speed and rotor speed, in other words: "the slip percentage expressed in RPM":

$$s_{rpm}=n_s-n_r$$