The principle that states that the energy of a system is conserved in a closed system is called the Law of Conservation of Energy. This law posits that energy cannot be created or destroyed but can only be transformed from one form to another. In various physical and chemical processes, while energy may change forms—such as from potential to kinetic energy—the total energy within a closed system remains constant. This principle underlies many scientific disciplines and is fundamental in understanding processes ranging from mechanics to thermodynamics.