Polymorphism means to take more than one form. function overloading and operator overloading are examples of compile time polymorphism and virtual function is an example of run time polymorphism.
Polymorphism is Poly+morph(multi form). It is implemented by method overloading and overriding. The JVM/compiler decide which method is to be called at runtime, this is called late binding. Polymorphism is a powerful mechanism of OOPS.