Nuclear fuel burnup is a fundamental metric quantifying the energy extracted from nuclear fuel per unit mass, reflecting the integrated effect of nuclear fission and subsequent isotopic changes over its residence time in a reactor core. As a research concept, it investigates the complex processes of fissile isotope depletion, the generation of fission products and transuranic elements, and the resulting evolution of fuel composition, reactivity, and physical properties. Characterized typically in units such as gigawatt-days per metric ton of heavy metal (GWd/tHM), burnup is of paramount significance in assessing nuclear fuel cycle efficiency, ensuring reactor safety, optimizing spent fuel storage and reprocessing strategies, and informing advanced reactor design.