The Web Ontology Language OWL and its successor OWL 2 are languages for defining and instantiating Web ontologies. Ontology is a term borrowed from philosophy that refers to the science of describing the kinds of entities in the world and how they are related. Entities, such as classes, properties, and individuals, are identified by URIs and can be thought of as primitive terms or names. Entities represent basic elements of the domain being modelled. For example, a class Person can be used to model the set of all people. The expressive power of the profile is necessarily quite limited, although it does include most of the main features of conceptual models such as UML class diagrams and ER diagrams. The QL acronym reflects the fact that query answering in this profile can be implemented by rewriting queries into a standard relational query language such as SQL.