Define slip speed?

is defined as the difference between the synchronous speed NS rpm and the actual speed N rpm .

It is define as the difference between rotor speed which is denoted (N) and the flux speed denoted by (Ns) 

The diference between synchronus speed and actual speed is slip speed

The Difference between synchronous spped and actual rotor speed in an inducton motor. this speed provides the basic principle of operation for induction machine.