Exergy is defined as the maximum work obtainable from a system as it reaches equilibrium with a heat reservoir, highlighting its significance in thermodynamics. It represents the useful energy available in a system, distinguishing it from total energy, which includes unusable energy. The concept is primarily applied in thermodynamic contexts, but it can also relate to other fields like engineering and environmental science. Understanding exergy helps in assessing energy efficiency and sustainability in various processes. Overall, exergy is crucial for evaluating how energy can be converted into work effectively.