Nuclear fusion is 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, where hydrogen nuclei fuse to create helium.
First theorized in the 1920s by Arthur Eddington who suggested that stars draw their energy from the fusion of hydrogen into helium. The physics was further developed by Hans Bethe in the 1930s.
While controlled fusion for commercial energy production is still in development, major international projects like ITER and advances at NIF (National Ignition Facility) have made significant breakthroughs. It promises a nearly limitless, clean energy source with no long-lived radioactive waste.