Loss approximation is the process of estimating or replacing a complex, computationally expensive, or mathematically intractable loss function with a simpler surrogate model that is easier to optimize or analyze. In machine learning and mathematical optimization, complex loss landscapes, such as non-convex objectives, are frequently approximated locally using simpler mathematical formulations like quadratic functions or Taylor expansions. This technique allows optimization algorithms to efficiently compute parameter updates, derive theoretical convergence guarantees, evaluate error bounds, and select representative data subsets while preserving the essential geometric properties of the underlying learning objective.