In philosophy, ontology is the study of being or existence, or the nature of being and kinds of existents; it comes from the Greek ont (to be) and -logy (theory of).
An ontology in AI refers to the set of kinds of objects and relations among objects that are represented in an AI system and used in reasoning; the ontology provides a vocabulary for talking about them.
One can have multiple ontologies, e.g. an ontology for set theory and an ontology for Newtonian mechanics.
http://www-ksl.stanford.edu/kst/what-is-an-ontology.html