1. <philosophy> A systematic account of Existence.
2. <artificial intelligence> (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. <information science> 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.
