fusion science definition

1. The joining together of atomic nuclei, especially hydrogen or other light nuclei, to form a heavier nucleus, especially a helium nucleus. Fusion occurs when plasmas are heated to extremely high temperatures, forcing the nuclei to collide at great speed. The resulting unstable nucleus emits one or more neutrons at very high speeds, releasing more energy than was required to fuse the nuclei, thereby making chain-reactions possible, since the reaction is exothermic. Fusion reactions are the source of the energy in the Sun and in other stars, and in hydrogen bombs. See also fission.
2. A mixture or blend formed by fusing two or more things.

fusion

nuclei of deuterium and tritium (two types of hydrogen atoms) joining by fusion to form a single helium atom and a free neutron