In computer science, a subtype states that if given type A is compatible with type B, then A is a subtype of B while not always vice versa. The most important distinction from mere datatype is one datatype can be more than one subtype. The definition is known as Liskov substitution principle.

In object-oriented programming, this means objects of the same subtype respond to the same messages.

A derived type is a type given a new type but structurally the same as the original type. The purpose of this type is create a new type name so that two values can have two distinct types in terms of name. It matters if the type system uses a name equivalence rule but does not matter if the system uses a structural equivalence.