Interoperability usually refers to software system communication. Although there is no widely accepted definition, and therefore no common understanding of the context, there are multiple solutions (protocols, architectures, components) that promise to solve integration issues. The INTEROP network of excellence aims at proposing a large view of interoperability issues, and hence requires a unified definition. As an INTEROP participant, we suggest in this paper, as a first attempt, an ontology of interoperability. We first present the general software engineering concepts our work is based on. We then propose the decisional meta-model and the technical