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Ontologies are developed to provide a machine-processable semantics of information
sources that can be communicated between different agents (software and humans).
Many definitions of ontologies have been given in the last decade, but one that, in our
opinion, best characterizes the essence of an ontology is based on the related definitions
in [Gruber, 1993]: 
An ontology is a formal, explicit specification of a shared
conceptualisation. 
A ‘conceptualisation’
refers to an abstract model of some
phenomenon in the world which identifies the relevant concepts of that phenomenon.
‘Explicit’
means that the type of concepts used and the constraints on their use are
explicitly defined. 
‘Formal’
refers to the fact that the ontology should be machine
readable. Hereby different degrees of formality are possible. Large ontologies like
WordNet provide a thesaurus for over 100,000 natural language terms explained in
natural language (see also [Meersman, 1999] for a discussion of this issue). On the other
end of the spectrum is CYC, that provides formal axiomating theories for many aspect
of common sense knowledge. 
‘Shared’
reflects the notion that an ontology captures
consensual knowledge, that is, it is not restricted to some individual, but accepted by a
group. Basically, the role of ontologies in the knowledge engineering process is to
facilitate the construction of a domain model. An ontology provides a vocabulary of
terms and relations with which to model the domain.