A reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as the release of large amounts of energy. This is the process that powers the sun and other stars.
First theorized in the 1920s by Arthur Eddington who suggested that stars draw their energy from the fusion of hydrogen into helium.
While controlled fusion for energy production is still in development, major international projects like ITER and NIF have made significant breakthroughs in achieving net energy gain.