noun

• (uncountable, philosophy) The branch of metaphysics that addresses the nature or essential characteristics of being and of things that exist; the study of being qua being.
• (uncountable, philosophy) In a subject view, or a world view, the set of conceptual or material things or classes of things that are recognised as existing, or are assumed to exist in context, and their interrelations; in a body of theory, the ontology comprises the domain of discourse, the things that are defined as existing, together with whatever emerges from their mutual implications.
• (countable, philosophy) The theory of a particular philosopher or school of thought concerning the fundamental types of entity in the universe.
• (logic) A logical system involving theory of classes, developed by Stanislaw Lesniewski (1886-1939).
• (countable, computer science, information science) A structure of concepts or entities within a domain, organized by relationships; a system model.