In the last years, important steps have been undertaken to bring the e-learning web to its full potential. In this paper, I describe an ontology that can serve as a further step in this direction. The ontology captures the instructional function of a learning resource, in other words, its ``essence'' from a teaching/learning perspective, an aspect not yet covered by learning object metadata standards. It offers the well-known advantages of ontologies: it can provide humans with a shared vocabulary and can serve as the basis for the semantic interoperability for machines. The article motivates the need for such an ontology and describes several educational Web services that can benefit from it. To exemplify the generality of the ontology, the article describes how the ontology can be mapped onto several knowledge representations currently used in e-learning systems.
