Subtyping

https://en.wikipedia.org/wiki/Subtyping

Symbols:

• <:, subtype: a <: b (a is a subtype of b)

• :>, supertype: b :> a (b is a supertype of a)

In programming language theory, subtyping is a form of type polymorphism in which a subtype is a datatype that is related to another datatype (its supertype) by some notion of substitutability; this means that a function expecting a value of the supertype can be given a value of the subtype instead.

Subtyping is also called subtype polymorphism or inclusion polymorphism.

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 depends on the specification of what safe use means in a given language).