nuclear fusion

action in which two light nuclei join into one stable nucleus which is lighter than the total of the two and therefore emits energy

In physics and nuclear chemistry, nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable. The fusion of two nuclei lighter than iron or nickel generally releases energy while the fusion of nuclei heavier than iron or nickel absorbs energy; vice-versa for the reverse process, nuclear fission. Nuclear fusion is naturally found in stars.
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Fusion /fu·sion/ 1. the act or process of melting. 2. the merging or coherence of adjacent parts or bodies. 3. [more]Nuclear fusion - Community and Resources

Noun
1. a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
(synonym) fusion, nuclear fusion reaction
(hypernym) nuclear reaction
(hyponym) cold fusion

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.