In knowledge representation and object-oriented programming and design, Is-a 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 "A is a B" usually means that concept A is a specialization of concept B, and concept B is a generalization of concept A.

For instance, a "fruit" is a generalization of "apple", "orange", "mango" and many others. One can say that an apple is a fruit.

In object-oriented programming the Is-a relationship arises in the context of inheritance concept. One can say that "apple" may inherit all the properties common to all fruits, such as being a fleshy container for the seed of a plant.

The Is-a relationship is contrasted with the Has-a relationship which constitutes a different kind of hierarchy.

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" or "Is-a".