Process by which two atomic nuclei join to make a heavier one. This is the type of nuclear reaction that takes place in the centre of stars, for instance in the fusion of hydrogen into helium. It occurs with light elements , of mass less than iron, and it releases energy (i.e.it is an exothermic reaction) because the mass of the product is less than the sum of the two fusing particles; the mass defect (lost mass) is turned into energy according the famous E = m c2 equation.
Astronomy and Physics Terms by ExploreSpace.com
A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy and, ultimately, of (almost) all energy on Earth.