ExpertGlossary

An ontology defines the common words and concepts (meanings) used to describe and represent an area of knowledge, and so standardizes the meanings. Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, counter terrorism, imagery, automobile repair, etc.). Ontologies include computer usable definitions of basic concepts in the domain and the relationships among them. They encode knowledge in a domain and also knowledge that spans domains. So, they make that knowledge reusable.

An ontology includes the following:

• Classes (general things) in the many domains of interest
• Instances (particular things)
• Relationships among those things
• Properties (and property values) of those things
• Functions of and processes involving those things
• Constraints on and rules involving those things

The open world assumption means that we cannot assume something doesn't exist until it is explicitly stated that it does not exist. In other words, because something hasn't been stated to be true, it cannot be assumed to be false - it is assumed that 'the knowledge just hasn't been added to the knowledge base'.

OWL-DL is much more expressive than OWL-Lite and is based on Description Logics (hence the suffix DL). Description Logics are a decidable fragment of First Order Logic2 and are therefore amenable to automated reasoning. It is therefore possible to automatically compute the classification hierarchy and check for inconsistencies in an ontology that conforms to OWL-DL.

OWL-Full is the most expressive OWL sub-language. It is intended to be used in situations where very high expressiveness is more important than being able to guarantee the decidability or computational completeness of the language. It is therefore not possible to perform automated reasoning on OWL-Full ontologies.

The syntactically simplest of the three sub-languages of OWL. It is intended to be used in situations where only a simple class hierarchy and simple constraints are needed. For example, it is envisioned that OWL-Lite will provide a quick migration path for existing thesauri and other conceptually simple hierarchies.

The class in an OWL ontology that represents the set containing all individuals. All classes in an OWL ontology are subclasses of owl:Thing.

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