Subsumption relation

In knowledge representation, object-oriented programming and design (see object oriented program architecture), is-a (is_a or is a) is a subsumption relationship between abstractions (e.g. types, classes), where one class A is a subclass  of another class B (and so B is a superclass  of A). In other words, type A is a subtype of type B when A’s specification implies B’s specification. That is, any object (or class) that satisfies A’s specification also satisfies B’s specification, because B’s specification is weaker.