The Collaborative International Dictionary of English v.0.48:

Ontology \On*tol"o*gy\, n. [Gr. ? the things which exist
   (pl.neut. of ?, ?, being, p. pr. of ? to be) + -logy: cf. F.
   ontologie.]
   1. That department of the science of metaphysics which
      investigates and explains the nature and essential
      properties and relations of all beings, as such, or the
      principles and causes of being.
      [1913 Webster]

   2. (Computers) A systematic arrangement of all of the
      important categories of objects or concepts which exist in
      some field of discourse, showing the relations between
      them. When complete, an ontology is a categorization of
      all of the concepts in some field of knowledge, including
      the objects and all of the properties, relations, and
      functions needed to define the objects and specify their
      actions. A simplified ontology may contain only a
      hierarchical classification (a taxonomy) showing the
      type subsumption relations between concepts in the field
      of discourse. An ontology may be visualized as an abstract
      graph with nodes and labeled arcs representing the objects
      and relations.

   Note: The concepts included in an ontology and the
         hierarchical ordering will be to a certain extent
         arbitrary, depending upon the purpose for which the
         ontology is created. This arises from the fact that
         objects are of varying importance for different
         purposes, and different properties of objects may be
         chosen as the criteria by which objects are classified.
         In addition, different degrees of aggregation of
         concepts may be used, and distinctions of importance
         for one purpose may be of no concern for a different
         purpose.
         [PJC]

WordNet (r) 3.0 (2006):

ontology
    n 1: (computer science) a rigorous and exhaustive organization
         of some knowledge domain that is usually hierarchical and
         contains all the relevant entities and their relations
    2: the metaphysical study of the nature of being and existence

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "ontology":
   aesthetics, axiology, casuistry, cosmology, epistemology, ethics,
   existentialism, first philosophy, gnosiology, logic,
   mental philosophy, metaphysics, moral philosophy, phenomenology,
   philosophastry, philosophic doctrine, philosophic system,
   philosophic theory, philosophical inquiry,
   philosophical speculation, philosophy, school of philosophy,
   school of thought, science of being, sophistry, theory of beauty,
   theory of knowledge, value theory



The Free On-line Dictionary of Computing (30 December 2018):

ontology

   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.

   (1997-04-09)