Source : Webster's Revised Unabridged Dictionary (1913)

Ontology \On*tol"o*gy\, n. [Gr. ? the things which exist
   (pl.neut. of ?, ?, being, p. pr. of ? to be) + -logy: cf.F.
   ontologie.]
   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.

Source : WordNet®

ontology
     n : the metaphysical study of the nature of being and existence

Source : Free On-Line Dictionary of Computing

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)