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In programming language theory, subtyping or subtype polymorphism is a form of type polymorphism in which a subtype is a datatype that is related to another datatype (the supertype) by some notion of substitutability. Substitutability means that program elements, typically subroutines or functions, written to operate on elements of the supertype can also operate on elements of the subtype.

In programming language theory, subtyping or subtype polymorphism is a form of type polymorphism in which a subtype is a datatype that is related to another datatype (the supertype) by some notion of substitutability. Substitutability means that program elements, typically subroutines or functions, written to operate on elements of the supertype can also operate on elements of the subtype.

If S is a subtype of T, the subtyping relation is often written S <: T, to mean that any term of type S can be safely used in a context where a term of type T is expected. The precise semantics of subtyping crucially depends on the particulars of what "safely used in a context where" means in a given programming language. The type system of a programming language essentially defines its own subtyping relation, which may well be trivial.