An ontology, also known as a semantic model, is a conceptualization of a domain; a definition of the concepts that are relevant to that domain. It is like a dictionary except that it is a formalization which allows the concepts defined in the ontology to have meaning for computers as well as for people. The concepts defined in the ontology become the domain-specific elements that can be used in other models, e.g., to write if/then rules, to write queries, to capture system requirements, etc.