The global energy system is in transition. It is attempting to reach net-zero greenhouse gas emissions by 2050. The systemic changes mean that the role of bioenergy will change. The potential of bioenergy to make a flexible contribution to the energy system is key for the achievement of global emission reduction ambitions and the functioning of the low-carbon energy system and economy. As the volume of sustainably available biomass resources is limited, defining the contributions from bioenergy to a low-carbon energy system and finding balances – and ideally synergies – between the different possible energy and climate system services that biomass can provide will be very important. The recognized system services include, among others, the flexible operation of bioenergy plants to integrate variable renewable energy sources and to provide negative carbon dioxide (CO₂) emissions. Interest in flexible operation of bioenergy value chains, bioenergy with carbon capture and utilization as well as synergies with renewable hydrogen-based value chains has increased recently. The objective of this paper is to present a holistic definition of flexible bioenergy as a system service based on the work conducted in International Energy Agency (IEA) Bioenergy Technology Collaboration Programme's Task 44 Flexible Bioenergy and System Integration, and to provide some practical examples. The paper also presents the different bioenergy system services and considers their definitions and interactions, as this is important in energy system design. The definition of flexible bioenergy shows that the flexibility provision from bioenergy goes far beyond the traditional definition of providing short-term flexibility in the power sector. Indicators to demonstrate the value of services as well as further quantitative assessment of synergies and trade-offs are needed to valorize the different services from bioenergy and create viable business cases.