We introduce the first formal rehabilitation robotics ontology, called REHABROBO-ONTO, to represent information about rehabilitation robots and their properties; and a software system REHABROBO-QUERY to facilitate access to this ontology. REHABROBO-QUERY is made available on the cloud, utilizing Amazon Web services, so that 1) rehabilitation robot designers around the world can add/modify information about their robots in REHABROBO-ONTO, and 2) rehabilitation robot designers and physical medicine experts around the world can access the knowledge in REHABROBO-ONTO by means of questions about robots, in natural language, with the guide of the intelligent user-interface of REHABROBO-QUERY. The ontology system consisting of REHABROBO-ONTO and REHABROBO-QUERY is of great value to robot designers as well as physical therapists and medical doctors. On the one hand, robot designers can access various properties of the existing robots and to the related publications to further improve the state-of-the-art. On the other hand, physical therapists and medical doctors can utilize the ontology to compare rehabilitation robots and to identify the ones that serve best to cover their needs, or to evaluate the effects of various devices for targeted joint exercises on patients with specific disorders.