An approximation and alternative-formalism concept defining methods for computing quantum predictions when exact solutions are impractical. It governs controlled expansions, action-based formulations, and phase-space representations that support analytic and numerical work. It does not ensure accuracy outside its regime of validity and requires explicit error assessment or convergence checks. It enables tractable estimates of spectra, transition rates, and dynamical behavior across a wide range of models. The concept is generally stable, though improved algorithms and convergence techniques evolve over time.