The word ontology is used with different meanings in different communities. We distinguish between Ontology (uncountable reading and capital initial) and an ontology (countable reading and lower-case initial). In the first case, we refer to a philosophical discipline, namely the branch of philosophy which deals with the nature and the organisation of reality. Unlike the special sciences, each of which investigates a class of beings and their determinations, Ontology regards all the species and tries to answer the question: What is being?, or What are the features common to all beings? In the second case, we refer to an information object and engineering artefact as the most prevalent use in the computer science communities.