[jonsen]

Any definition of polymorphism should at least mention something about the method that is invoked depends on runtime data, and is not resolved (or known) at compile time.

Yes

  class A { ... f() ... }
  class B extends A { ... f() ... }
  class C extends A { ... f() ... }
  A x =  ...  // a B object or a C object
  x.f()

is polymorphism if anything.