In terms of Java, when someone asks:
what is polymorphism?
Would overloading or overriding be
Polymorphism simply means "Many Forms".
It does not REQUIRE inheritance to achieve...as interface implementation, which is not inheritance at all, serves polymorphic needs. Arguably, interface implementation serves polymorphic needs "Better" than inheritance.
For example, would you create a super-class to describe all things that can fly? I should think not. You would be be best served to create an interface that describes flight and leave it at that.
So, since interfaces describe behavior, and method names describe behavior (to the programmer), it is not too far of a stretch to consider method overloading as a lesser form of polymorphism.