[Significance] The development of urban underground space faces severe geotechnical safety challenges, with disaster origins largely stemming from the hazard-transformation evolution of specific geological bodies under engineering disturbances. [Progress] The conventional engineering concept of “subsurface defects” (e.g., cavities, loosened zones, water-rich bodies) focuses on the manifestations of hazards, making it difficult to support risk control at the source. Meanwhile, existing  geological concepts, while capable of characterizing objective geological units, fail to adequately represent their dynamic response and disaster- inducing potential under engineering activities. [Conclusions and Prospects] To bridge the theoretical gap between “geological conditions” and “engineering hazards”, this paper proposes the core concept of the “Geological Hazard Body”, defining it as “a specific geological unit that may evolve into an engineering hazard under natural or anthropogenic disturbances”. On this basis, a classification system with dual criteria of genetic origins and disaster-forming mechanisms is established, systematically covering main types such as rock masses, soil masses, groundwater bodies, and geological structures, while also elucidating their evolution pathways toward engineering-scale hazards. The conceptual system established in this study provides a unified conceptual framework for promoting a paradigm shift in risk perception from “phenomenon response” to “root cause management.” It also lays a taxonomic foundation for subsequent research on dynamic simulation and quantitative assessment of geological hazard bodies.