Specialization

Concept B is a specialization of concept A if and only if:

• every instance of concept B is also an instance of concept A; and
• there are instances of concept A which are not instances of concept B.

Equivalently, B is a specialization of A iff A is a generalization of B.

For instance, 'Bird' is a specialization of 'Animal' because every bird is an animal, and there are animals which are not birds (humans, for instance).