Pervasive conceptual ambiguity and inconsistent application of the term “environmental sustainability” obstruct coherent policy and practice in the energy sector, often resulting in greenwashing, fragmented regulations, and unmet ecological goals. This chapter directly addresses this critical problem by constructing a rigorous, sector-specific conceptual foundation essential for meaningful progress. It first establishes the non-negotiable imperative for sustainability within energy systems, anchoring the discussion in global climate accords, planetary boundaries, and long-term energy security. The analysis also critically synthesises core definitions, distilling a precise, operational framework that transcends generic usage to inform energy production, distribution, and consumption. This framework is applied to systematically catalog and evaluate the multidimensional environmental impacts across the entire energy spectrum; i.e. from the localised pollution of fossil fuels to the lifecycle challenges of renewable technologies. Finally, the chapter transitions from definition to action, and outlines integrated pathways for achieving sustainability through principles of ecological integrity, radical resource efficiency, and circular economy models. By providing this essential conceptual clarity, the chapter arms stakeholders with the intellectual toolkit needed to design, govern, and transition towards energy systems that genuinely reconcile human development with the imperative of ecological preservation.