Definition of "ontology"

ontology

/ɒnˈtɒləd͡ʒi/

noun
countable and uncountable, plural ontologies
1. (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.
  Quotations
  [Martin] Heidegger's concern […] was with ontology, the nature of beings, above all humans. The central question for him was "What is being? What is it for something to be?" He tackled this question not by way of the sciences, but by way of an examination of our prescientific daily life. We are, he argued, not cut off from the world by our mental processes: we are "in the world", in direct contact with our surroundings.
  2014 April 12, Michael Inwood, “Martin Heidegger: the philosopher who fell for Hitler [print version: Hitler's philosopher]”, in The Daily Telegraph (Review)‎, London, page R10
2. (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.
  Quotations
  Do you believe in ghosts? Then ghosts are in your ontology, along with tables and chairs and songs and vacations, and snow, and all the rest. It has proved more than convenient to extend the term "ontology" beyond this primary meaning and use it for the set of "things" that an animal can recognize and behave appropriately with regard to (whether or not animals can properly be said to have beliefs) and — more recently — the set of "things" a computer program has to be able to deal with to do its job (whether or not it can properly be said to have beliefs). Vacations are not in the ontology of a polar bear, but snow is, and so are seals. Snow is probably not in the ontology of a manatee, but outboard-motor propellers may well be, along with seaweed and fish and other manatees. The GPS system in your car handles one-way streets, left and right turns, speed limits, and the current velocity of your car (if it isn't zero, it may not let you put in a new target address), but its ontology also includes a number of satellites, as well as signals to and from those satellites, which it doesn't bother you with, but needs if it is to do its job.
  2017, Daniel C. Dennett, From Bacteria to Bach and Back
3. (countable, philosophy) The theory of a particular philosopher or school of thought concerning the fundamental types of entity in the universe.
  Quotations
  The answer to the controversial question of whether Aristotle's ontology includes non-substantial particulars, then, is that it does.
  2000, C. D. C. Reeve, Substantial Knowledge: Aristotle's Metaphysics, Hackett Publishing, page 97
4. (logic) A logical system involving theory of classes, developed by Stanislaw Lesniewski (1886-1939).
5. (countable, computer science, information science) A structure of concepts or entities within a domain, organized by relationships; a system model.