Nuclear fusion was a human term describing the process by which multiple atomic nuclei joined together to form a heavier nucleus. This was accompanied by the release or absorption of energy. In the usual case, the energy released was such as to raise the temperature of the matrials invovled to very high levels, ionizing all of the atoms and producing a plasma state.
Fusion occurred naturally in stars. Controlled fusion for human purposes was also achieved, although it was initially rather difficult to control. Because hydrogen was widely available, and hydrogen fusion could produce sufficient energy to power starships and other power needs in an advanced civilization, fusion technology was widely used in both weapons and civilian devices, mainly power plants.
However, on Grayson, fission plants were used (HH2) prior to its rediscovery by Galactic civilization, since fusion techniques had been lost. Following rediscovery Grayson adopted fusion technology widely including use in hyper-capable warships such as GNS Saul and Covington. But pre-alliance Grayson fusion plants were four times as massive as modern reactors of similar output and fissionable materials were plentiful, so Grayson did not abandon its fission technology. It was useful in developing some new classes of warships. When the Manticoran Alliance introduced the new LACs in the end of the First Havenite-Manticoran War they were powered by fission plants, too. (HH8)