On Ontologies and Knowledge Sharing from the (free) book on Artifical Intelligence – Foundations of Computational Agents:
- a vocabulary of the categories of the things (both classes and properties) that a knowledge base may want to represent;
- an organization of the categories, for example into an inheritance hierarchy using subClassOf or subPropertyOf, or using Aristotelian definitions; and
- a set of axioms restricting the meanings of some of the symbols to better reflect their meaning – for example, that some property is transitive, or that the domain and range are restricted, or that there are some restriction on the number of values a property can take for each individual. Sometimes relationships are defined in terms of more primitive relationships but, ultimately, the relationships are grounded out into primitive relationships that are not actually defined