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.
R. Prieto-Díaz. Information Reuse and Integration, 2003. IRI 2003. IEEE International Conference on, page 458-465. IEEE Systems, Man, and Cybernetics Society, (2003)