In spite of the growing interest in the term, the meaning of the word ontology is not easy to define or grasp, nor is it used in the same way by different people. The term is borrowed from philosophy where it is used to describe the study of reality and objects independently of our knowledge of them.
In application, a necessary characteristic of an ontology is that it expresses or embodies some sort of world view, i.e. a perspective, a way of thinking about or carving up of some aspect of the natural or artificial world. It might be thought of it as a model or paradigm. The world view is often conceived as a set of concepts (e.g. entities, attributes, processes etc.), their definitions, and their inter-relationships [9].
Any piece of application software will reflect a world view of the application domain. For example, an accounting package embodies a generic model of accounting concepts (e.g. an invoice; a department in an organisation.) Normally, this world view is implicit, and exists only in the heads of the people designing and/or using the software. In the case of accounting software, the world view is also expressed in any number of textbooks. However, exactly which world view is it? Problems arise when similar terms are used in different ways. There is no one single universally agreed accounting world view, though there may be a core of concepts that most people agree about. Because of this, ambiguities may arise and it can thus be useful to record explicitly the world view that the software is based on.
The ontology offers the vocabulary for input and output. An ontology implies a high degree of buying-in to the definitions contained in it on the part of its users.
Ontologies have been derived within the Knowledge Sharing Effort [10] for areas such as bibliographic data, configuration design, job assignment, numbers, sets and time.