Ontology

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.

from http://www.dictionary.com/cgi-bin/dict.pl?term=ontology

-- MartinCleaver - 26 Mar 2001