What is an Ontology?

An ontology is a model of a domain of knowledge, describing the concepts within the domain and the relationships between concepts in the domain.

Modelling Knowledge

The purpose of an ontology is to model a domain of knowledge, such as medical knowledge, football knowledge or universal knowledge. This model provides a very rich source of information to applications using the ontology. It also provides a separation of this knowledge from the application such that many and varied applications can use the same information.

The building blocks of an ontology are concepts. Concepts are abstract representations of the fundamental things you are modelling in your domain. For example, in the ontology of medical knowledge our concepts may be antibiotics, ulcers, aspirin, etc.

The power of the ontology is realised through associations. Associations define binary relationships between concepts. For example we may say that drugs are related to symptoms, in that the former treat the latter. In this definition of an association drug and symptom are called roles. We can show this association definition using a simple diagram:

Once we've defined an association in this way we can define actual instances of the association. These are called links. Here is an example:

Here are some more examples in a shorthand notation:

• Treatment: Aspirin Headache
• Treatment: Aspirin Fever
• Treatment: Antibiotics Bacterial Diseases
• Treatment: Hydrocortisone Itching
• Side-effect: Aspirin Stomach Ulcer
• Drug branding: Aspirin Disprin