Cyber-physical systems (CPS) usually have to cope with open contexts, i.e. the context that these systems interact with through sensors and actuators or communication networks changes its morphology during runtime. For instance, CPS can be deployed in the manufacturing domain to form a modular production system, which, at some time during its lifecycle, might have to produce new products or to interact with new modules, which have been designed by different stakeholders. In order to be able to cope with open contexts, CPSs rely on ontologies that represent the information being shared among them. However, due to the complexity of a CPS's context, building these ontologies as a basis for information exchange during runtime is a non-trivial task that requires a structured approach. The contribution of this paper is twofold. First, we present an ontology building method that is tailored towards the needs of CPS, beginning with ontological requirements and ending with a formal ontology. Second, we apply this method to build an ontology for communicating information between CPSs about processes and machine states in the manufacturing domain.