The Ontology Problem is a fundamental challenge of the emerging Semantic Web. This problem is comprised of three key sub-problems, the Upper Ontology Problem, the Domain Ontology Problem, and the Ontology Integration Problem.
A. Salminen, and M. Virtanen. Proceedings of the 7th International Conference on Enterprise Information Systems, ICEIS 2005, page 468-473. INSTICC Press, (2005)