Pr4 UG co Ontology
An ontology is a formally-defined vocabulary for a particular domain of interest. Because it is formally defined, it can reduce the opportunity for confusion in human discourse. Indeed, some ontologies are created for this exact purpose.
However, ontologies are typically created to enable a computer program (eg, a reasoner) to infer logical consequences about the domain. So, developing an ontology might be a precursor to creating a database, expert system, or knowledge base.
The restriction to "a particular domain of interest" may seem odd. However, there are both practical and theoretical reasons for limiting the subject matter and the encoded information. In brief, the problem is that the meaning of a term depends on the kinds of questions being asked.
For example, even something as prosaic as an onion will be seen in different ways to a biologist, a cook, a farmer, and a grocer. Details which are critical to one person may be completely irrelevant to another.
An OWL ontology (as used in Protege 4) is based on a class hierarchy. Each class defines a level of abstraction. For example, the Pizza Ontology defines classes such as Pizza, VegetarianPizza, and Margherita.
The hierarchy is then "decorated" with properties. Each property specifies a binary (ie, dyadic, two-ended) relationship. A relationship might indicate that a Pizza has:
- a PizzaBase,
- one or more PizzaToppings, or
- certain standardized sizes.