the branch of metaphysics that studies the nature of existence or being as such.
(loosely) metaphysics.

1715–25; < Neo-Latin ontologia. See onto-, -logy

ontological [on-tl-oj-i-kuhl] , ontologic, ontologistic [on-tol-uh-jis-tik] , adjective
ontologist, noun
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
ontology (ɒnˈtɒlədʒɪ)
1.  philosophy the branch of metaphysics that deals with the nature of being
2.  logic the set of entities presupposed by a theory

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

"metaphysical science or study of being," 1721, from Mod.L. ontologia (coined in Fr. by Jean le Clerc, 1692), from Gk. on (gen. ontos) "being" (prp. of einai "to be;" see essence) + -logia "writing about, study of."
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

ontology definition

1. A systematic account of Existence.
2. (From philosophy) An explicit formal specification of how to represent the objects, concepts and other entities that are assumed to exist in some area of interest and the relationships that hold among them.
For AI systems, what "exists" is that which can be represented. When the knowledge about a domain is represented in a declarative language, the set of objects that can be represented is called the universe of discourse. We can describe the ontology of a program by defining a set of representational terms. Definitions associate the names of entities in the universe of discourse (e.g. classes, relations, functions or other objects) with human-readable text describing what the names mean, and formal axioms that constrain the interpretation and well-formed use of these terms. Formally, an ontology is the statement of a logical theory.
A set of agents that share the same ontology will be able to communicate about a domain of discourse without necessarily operating on a globally shared theory. We say that an agent commits to an ontology if its observable actions are consistent with the definitions in the ontology. The idea of ontological commitment is based on the Knowledge-Level perspective.
3. The hierarchical structuring of knowledge about things by subcategorising them according to their essential (or at least relevant and/or cognitive) qualities. See subject index. This is an extension of the previous senses of "ontology" (above) which has become common in discussions about the difficulty of maintaining subject indices.
The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Example sentences
Nature, the brute fact of existence the ultimate ontology, prevails.
All of the information is arranged in a complicated ontology.
Things become easier by abandoning ontology altogether.
Metaphysical naturalism represents a particular view about reality and hence
  belongs to the philosophical field of ontology.
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