The interoperability of educational systems, encouraged by the development of specifications, standards and tools related to the Semantic Web is limited to the exchange of information in domain and student models. High system interoperability requires that a common framework be defined that represents the functional essence of educational systems. To address this need, we developed a generic model of educational systems that we called Cognitive Ontology of Educational Systems (COES) and we encoded it as functional reference ontology. It generalizes the educational system architecture, taking into account the cognitive perspective. This framework ranges from the usual e-learning systems to complex cognitive and adaptive hypermedia systems. This article describes the part of the COES related to domain knowledge representation and proposes an implementation called Generic Educational Knowledge (GEK). The GEK model is structured in a flexible way that allows the authors to codify instructional and semantic levels as needed. In order to test its feasibility, the model was applied in a distance learning course using two educational-knowledge models, GEK and ADLSCORM, and two educational systems, a traditional Web-Based Adaptive Hypermedia System and a Rich Internet Application with dialog interaction and cognitive monitoring. (Contains 13 figures.)