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The Collaborative International Dictionary of English v.0.48 (gcide)

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]