The performance continuum for air-breathing engines is formally developed and illustrated in terms of fundamental thermodynamic quantities including heat and work interactions and the irreversibility occurring in the flow-path of the engine. The thermodynamically consistent base-line from which performance losses due to irreversibility must be measured is clearly defined based on this analysis. Issues and problems with conventional flow availability (flow exergy) in terms of the assessment (design and optimization) of jet engines are discussed. The formal analytical relationship between lost thrust work and the irreversible generation of entropy in a jet engine is then reviewed in terms of underlying principle and methodology used to quantify this lost thrust work. This relationship is then extended based on the same underlying principle to the more general concept of lost thermodynamic work across a jet engine. It is then proposed that this concept of lost thermodynamic work as measured between the actual and the reversible device (rather than as referenced to a thermodynamic dead state) can, in fact, be extended to encompass other sub-systems and ultimately can be applied across the overall aerospace vehicle.