Direct action is a concept within nuclear physics and astrophysics referring to the fundamental physical processes by which atomic nuclei undergo transformations. These processes, such as nuclear fusion and fission, directly result in the release or conversion of energy. As an academic concept, it investigates the intrinsic mechanisms, conditions, and energetic outcomes of these core nuclear reactions without external mediation. It is characterised by focusing on the immediate nuclear interactions and their direct energetic consequences. Its significance lies in explaining fundamental astrophysical phenomena like stellar energy generation and informing the development of nuclear energy technologies.