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.
S. Jeschke, M. Wilke, N. Natho, and O. Pfeiffer. Hawaii International Conference on System Sciences, Proceedings of the 41st Annual, page 330-330. (2008)