Is-a

In knowledge representation, object-oriented programming and design, is-a or is_a or is a (subsumption) is a relationship 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.
The is-a relationship is contrasted with the has-a relationship, which constitutes the holonym-meronym hierarchy. It may also be contrasted with the instance-of relation: see type-token distinction. When designing a model (e.g., a computer program) of the real-world relationship between an object and its subordinate, a common error is confusing the relations has-a and is-a.