Why is an induction motor called an asynchronous motor?

Induction motors are called asynchronous motors because the speed of their rotor is different from the speed of the rotating magnetic field produced by the stator. Specifically, when the rotating magnetic field generated by the stator (whose speed is the synchronous speed n1) moves relative to the rotor winding, the rotor winding cuts the magnetic lines of force, thus generating an induced electromotive force, which in turn causes an induced current in the rotor winding. 

This induced current interacts with the magnetic field, producing an electromagnetic torque that makes the rotor start to rotate. However, as the rotor speed gradually approaches the synchronous speed, the induced current will gradually decrease, and the resulting electromagnetic torque will also decrease accordingly. Therefore, when the induction motor works in the motor state, the actual speed of the rotor is always less than the synchronous speed. This speed difference is defined as the slip rate (slip), and it is precisely because of this slip that the working state of the induction motor differs from that of the synchronous motor, hence the name "asynchronous motor".