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 (written as , , or ) means that any term of type S can safely be used in any context where a term of type T is expected. The precise semantics of subtyping here crucially depends on the particulars of how "safely be used" and "any context" are defined by a given type formalism or programming language.