This paper proposes a generic, extensible, and scalable definition of hybrid energy storage systems (HESS) and provides a corresponding information model applicable for energy management system (EMS) implementation. Given the need for flexibility in both energy supply and demand due to the energy transition, multiple energy carriers have been coupled, energy storage mediums have been leveraged, and their characteristics have been optimized. EMS are adapting to these developments, which can be facilitated by having common definitions and information models. There are at least two prevailing descriptions of HESS: one based on complementary characteristics, and another based on the constituent energy storage mediums. The proposed definition is an extension and specific application of the concept of "energy hubs" and a clarification of the multiple descriptions of HESS. On a larger scale, this work aims to facilitate the interoperability of various EMS that involve HESS and to provide a foundational resource for projects related to HESS architectures, control, and optimization. This work is a contribution to the development of an open ontology tailored for EMS applications in the context of energy communities with HESS.