What is slip of an induction motor?

the difference between the flux speed (Ns) and the rotor speed (N). 

Slip is defined as the difference between synchronous and actual speed. If a motor runs at synchronous speed, it will not be able to induce rotor current, so slip is necessary in induction motors so they may run at slightly slower than synchronous speed but maybe able to induce rotor current.

Slip is the difference between actual and synchronous speed 

LIP is the difference between the synchronous speed of the magnetic field and the shaft rotating speed and would be some number of RPM or frequency