Ontology modeling requires modeling languages expressive enough to represent various definition types. A definition type which seems to be of particular significance is that provided by the Classical Theory of Definition. In this paper we investigate if and how far the Classical Theory of Definition is adopted by some of the ontology modeling formalisms, namely by UML, ORM and DL. Moreover, we provide a means for representing some crucial issues in the context of the Classical Theory of Definition which seem to have no representation in the formalisms discussed. Among them are the identification of essential, peculiar and incidental predications and the representation of subsumption in the manner of the genus-differentia definition.