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. Hotho, S. Staab, and G. Stumme. Proceedings of the 2003 IEEE International Conference on Data Mining, page 541-544 (Poster. Melbourne, Florida, IEEE Computer Society, (November 2003)