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An Ontology is a set of concepts or categories within one subject matter or domain that show properties of entities as well as their relationships to one another. In the case of knowledge graphs, ontologies allow data to be semantic.
In the case of Diffbot’s Knowledge Graph™ different entity types have different ontologies. For example, an article entity may include a “date posted on” field. While this field would not be applicable to people or business entities. 
Additionally, ontologies often provide a set of rules dictating a hierarchy among entity types. For example, a product is part of a brand, and a brand is part of an organization. An organization entity may have posted an article. But it does not make sense for an article to have possession of an organization. These rules allow automated systems to infer many facts as well as provide for intuitively explorable relationships between entities.