Polymorphism is the ability of an object (or reference) to assume (be replaced by) or become many different forms of object. Inheritance (or delegation) specifies slightly different or additional structure or behavior for an object, and these more specific or additional attributes of an object of a base class (or type) when assuming or becoming an object of a derived class characterizes object-oriented polymorphism. This is a special case of parametric polymorphism, which allows an object (or reference) to assume or become any object (possibly satisfying some implicit or explicit type constraints (parametric type), or a common structure), with this common structure being provided by base classes or types (subclass and subtype polymorphism, respectively).
"Poly" means "many" and "morph" means "form". The homograph polymorphism has many uses in the sciences, all referring to objects that can take on or assume many different forms. Computer Science refers to Strachey's original definitions of polymorphism, as divided into two major forms, parametric and ad-hoc. Cardelli and Wegner followup with another classification scheme, adding inclusion polymorphism for subtyping and inheritance.
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