What is a Fuel?

Chemically fuelled reactions are central to supramolecular chemistry, enabling transient assemblies, molecular machines, and non-equilibrium states. Yet, the term fuel remains inconsistently defined. In this perspective, we explore what qualifies as a fuel through the lens of classical physics and thermodynamics, distinguishing between any exergonic reactants - termed a 'trivial fuel' - and those that actively maintain a system away from equilibrium. We propose that a chemically fuelled reaction involves a coupled process: a primary transformation (R -> P) linked to a separate fuel-to-waste (F -> W) reaction. This coupling imparts a kinetic asymmetry, displacing the system from equilibrium and generating a non-equilibrium steady state (NESS). By analysing reaction-free energy landscapes and drawing analogies to Newtonian mechanics, we offer a unified framework for interpreting fuels as reagents that supply energy to resist equilibration. Co-authored by Stefan Borsley, this piece compares definitions from leaders in the field and emphasises the importance of mechanistic clarity in describing fuelled systems. While we acknowledge differing views, we find value in retaining the term fuel for its conceptual richness and evocative power. We hope this discussion stimulates more precise thinking - and creative chemistry - in the design of future fuelled systems.