Current research in health informatics should provide the techniques and tools that will enable the development of an efficient hospital information ecosystem. The different information systems (IS) will be able to efficiently communicate with each other and provide a patient oriented environment. It is thus important to provide a clear understanding of this ecosystem during the analysis phase. The solution is to develop a hospital information systems ontology that will provide the infrastructure for a clear understanding of this ecosystem and thus lead to the development of systems that will be able to work efficiently with each other. This ontology is developed here and its value is demonstrated through a case study.