19.2.2 Hyponymy

One sense is a hyponym of another sense if the first sense is more specific, denoting a subclass of the other. For example, car is a hyponym of vehicle; dog is a hyponym of animal, and mango is a hyponym of fruit. Conversely, we say that vehicle is a hypernym of car, and animal is a hypernym of dog. It is unfortunate that the two words (hypernym and hyponym) are very similar and hence easily confused; for this reason the word superordinate is often used instead of hypernym.

We can define hypernymy more formally by saying that the class denoted by the superordinate extensionally includes the class denoted by the hyponym. Thus the class of animals includes as members all dogs, and the class of moving actions includes all walking actions. Hypernymy can also be defined in terms of entailment. Under this definition, a sense A is a hyponym of a sense B if everything that is A is also B and hence being an A entails being a B, or $ \forall x A(x) \Rightarrow B(x) $. Hyponymy is usually a transitive relation; if A is a hyponym of B and B is a hyponym of C, then A is a hyponym of C.

The concept of hyponymy is closely related to a number of other notions that play central roles in computer science, biology, and anthropology and computer science. The term ontology usually refers to a set of distinct objects resulting from an analysis of a domain, or microworld. A taxonomy is a particular arrangement of the elements of an ontology into a tree-like class inclusion structure. Normally, there are a set of well-formedness constraints on taxonomies that go beyond their component class inclusion relations. For example, the lexemes hound, mutt, and puppy are all hyponyms of dog, as are golden retriever and poodle, but it would be odd to construct a taxonomy from all those pairs since the concepts motivating the relations is different in each case. Instead, we normally use the word taxonomy to talk about the hypernymy relation between poodle and dog; by this definition taxonomy is a subtype of hypernymy.