The term ontology is used in philosophy to describe efforts to understand and explain what it means for something to exist. It is constructed from Greek roots (ontos, which means “being”). In information systems, the term is used with a different meaning. Here, ontology describes the conceptualization of a “part of reality.” Unlike ontology in philosophy, which tries to uncover the true laws of being, ontologies in information science describe a particular way to understand a part of the world. Ontology is an important theoretical foundation for geographic information science, especially to achieve interoperability. This entry explores the roots of the term in philosophy and then moves on to discuss its role in geographic information science.