Is-a:
In knowledge representation and object-oriented programming and design, is-a is a relationship where one class D is a subclass of another class B (and so B is a superclass of D).
In other words, "D is a B" usually means that concept D is a specialization of concept B, and concept B is a generalization of concept D. 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.
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