What is Ontology

Although it is required from an ontology to be formally defined, there is no common definition of the term "ontology" itself. The definitions can be categorized into roughly three groups:

Ontology is a term in philosophy and its meaning is ``theory of existence''.
Ontology is an explicit specification of conceptualization.
Ontology is a body of knowledge describing some domain, typically common sense knowledge domain.

The definition 1 is the meaning in philosophy as we have discussed above, however it has many implications for the AI purposes. The second definition is generally accepted as a definition of what an ontology is for the AI community. The last third definition views an ontology as an inner body of knowledge, not as the way to describe the knowledge.