The concept of hazard has been extensively used in the literature and defined in an informal way, which serves as a guidance on identifying the potential hazards during the development of safety-critical systems. Intuitively, the definitions seem to be consistent and easy to understand. However, when we take a closer look at these definitions, ambiguities may arise, and real-world semantics need to be defined. In this work, we propose a hazard domain ontology, ie, the Hazard Ontology (HO), to provide an ontological interpretation of hazard. To tackle the aforementioned issues, the HO is grounded in the Unified Foundational Ontology (UFO) to utilize the benefits provided by taking foundational concepts into account. Finally, we show some useful findings when we use the proposed ontology to analyze the hazard descriptions from an industrial passenger train project.