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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. 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]