From 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.
   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
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