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. de Almeida Falbo, G. Guizzardi, and K. Duarte. SEKE '02: Proceedings of the 14th international conference on Software engineering and knowledge engineering, page 351--358. New York, NY, USA, ACM Press, (2002)