Processes in which two lighter atomic nuclei merge to form a more massive nucleus. For nuclei lighter than those of iron, energy is released in fusion. This is the main source of energy of ordinary stars like our sun.
Some more information about nuclear fusion can be found in the spotlight topic Is the whole the sum of its parts?. The role played in nuclear fusion and fission by Einstein’s famous formula E=mc² is the subject of the spotlight topic From E=mc² to the atomic bomb.