Polymorphism is a feature.
In programming languages and type theory, polymorphism is the provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types.The most commonly recognised major classes of polymorphism are: Ad hoc polymorphism: defines a common interface for an arbitrary set of individually specified types. Parametric polymorphism: when one or more types are not specified by name but by abstract symbols that can represent any type. Subtyping (also called subtype polymorphism or inclusion polymorphism): when a name denotes instances of many different classes related by some common superclass.. Read more on Wikipedia...
Languages without Polymorphism include Jule
Languages with Polymorphism include JavaScript, Pascal, Elixir, Felix
"a" + "b"; 1 + 2
https://hexdocs.pm/elixir/Protocol.html
// overloads
fun f (x:double) => x +42.1;
fun f (x:int) => x + 1;
fun f (x:string) => x + "!";