Current researches in health informatics provide the techniques and tools enabling the development of efficient hospital information ecosystem that can be made to exchange data with each other and thus provide a coherent patient oriented environment. In order to achieve this desired HIS interoperability, it is important to provide a clear understanding of the HIS ecosystem during the analysis phase that will provide the infrastructure of this ecosystem and lead to the development of systems that will be able to correspond with each other. Such an ontology is described here and its value will be demonstrated through a case study.