Ontology & Ontologies* [1]
An ontology defines the terms used to describe and represent an area
of knowledge.
Ontologies are used by people, databases, and applications that need to
share domain information (a domain is just a specific subject area or
area of knowledge, like medicine, tool manufacturing, real estate,
automobile repair, financial management, etc.).
Ontologies include computer-usable definitions of basic concepts in
the domain and the relationships among them … They encode
knowledge in a domain and also knowledge that spans domains. In this
way, they make that knowledge reusable.
The word ontology has been used to describe artifacts with different
degrees of structure. These range from simple taxonomies (such as the
Yahoo hierarchy), to metadata schemes (such as the Dublin Core), to
logical theories. The Semantic Web needs ontologies with a significant
degree of structure.