The Semantic Web - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 27

3.1.2 Generality of Ontologies

An ontology is a specification of a shared conceptualization. Therefore, domain
experts, users, and designers need to agree on the knowledge specified in an
ontology so that the ontology may be shared and reused. It is hard to get such
agreement. It is therefore advantageous to layer the knowledge in different
ontologies on the basis of generality, so that not everybody needs to agree
to all ontologies. Agreement is required only between specific domain and
application ontologies and between the higher-level ontologies that are being
used [91].
In the literature [38, 56, 62, 125], we generally find three common layers
of knowledge. On the basis of their levels of generality, these three layers
correspond to three different types of ontologies, namely:

• Generic (or top-level) ontologies, which capture general, domain indepen-

dent knowledge (e.g. space and time). Examples are WordNet [37] and
CYC [82]. Generic ontologies are shared by large numbers of people across
different domains.

• Domain ontologies, which capture the knowledge in a specific domain. An

example is UNSPSC, 3 which is a product classification scheme for vendors.
Domain ontologies are shared by the stakeholder in a domain.

• Application ontologies, which capture the knowledge necessary for a spe-

cific application. An example could be an ontology representing the struc-
ture of a particular Web site. Arguably, application ontologies are not
really ontologies, because they are not really shared.

The separation between these three levels of generality is not always strict.
WordNet, for example, contains some domain-specific relations and CYC con-
tains domain-specific microtheories (modules of the ontology).
Although sometimes other types of ontologies, such as representational on-
tologies or task ontologies are distinguished, the above three types of ontolo-
gies are common in the literature and are, in our opinion, a useful separation
of types of ontologies along the dimension of generality.

3.1.3 Expressiveness of Ontologies

Orthogonal to the generality of ontologies is their expressiveness. We distin-
guish the following levels of expressiveness (partly on the ontology spectrum
introduced in [89]):

• Thesaurus. Relations between terms, such as synonyms, are additionally

provided. Again, WordNet [37] is an example.

3 http://www.unspsc.org .