Concept A is a generalization of concept B 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, A is a generalization of B if B is a specialization of A.
For instance, animal is a generalization of bird because every bird is an animal, and there are animals which are not birds (dogs, for
instance).
This kind of generalization versus specialization (or particularization) is reflected in either of the contrasting words of the word pair hypernym and hyponym. A hypernym as a generic stands for a class or group of equally-ranked items, such as tree does for beech and oak; or ship for cruiser and steamer. Whereas a hyponym is one of the items included in the generic, such as lily and daisy are included in flower, and bird and fish in animal. A hypernym is superordinate to a hyponym, and a hyponym is subordinate to hypernym.
See also