In this introduction, we will use the meaning of ontology as an "explicit specification of conceptualization". The ontology in this sense constrains the intended usage of the terms in ontology (as illustrated in the figure in the previous page) so that the ontology forms a vocabulary and axioms that can be used to express knowledge base and that can be used for sharing knowledge between different systems.
In addition, we require that the specification is formal (like a program is formally written in a programming language), so that the ontology can be processed by a computer. Some of the formal languages that can be used for expressing ontologies are described in the next chapter.
Unless said otherwise, in the rest of this introduction the ontology is used in the sense of "formal explicit specification of conceptualization".