ontology

Ontology is the branch of philosophy that investigates the nature of being, existence, and the basic categories of what exists. It asks what kinds of entities populate the world, how they can be grouped into categories, and how these categories relate to one another. Core topics include substance, properties, relations, time, space, identity, universals versus particulars, and the differences between necessary and contingent existence. Ontology also engages with questions about realism, nominalism, and how best to describe the structure of reality.

Historically, ontology has its roots in ancient Greek philosophy and was developed through medieval scholasticism and modern metaphysics. In the 20th century, analytic philosophy further refined ontological questions, addressing issues such as the nature of universals, the limits of classifying beings, and the role of ontological commitment in theories.

In information science and artificial intelligence, the term ontology has a more technical sense. An ontology is a formal specification of a shared conceptualization for a domain. It defines concepts (classes), properties (relations), and constraints (axioms) and supports reasoning and interoperability. Ontologies are expressed in formal languages such as OWL or RDF and are used in knowledge representation, data integration, semantic search, and natural language understanding. Notable examples include the Gene Ontology in biology and various medical or domain-specific ontologies.

Although the philosophical and practical uses differ, both senses address how beings and their categories are understood and organized.