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 materials involved 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.
Royal Manticoran Navy starships, and most other navies used several fusion plants to power their systems. (HH1) The Isler Corporation was a large-scale producer of fusion reactors for the RMN. (JIR1)
On Grayson, fission plants were in common use (HH2) prior to its rediscovery by Galactic civilization, since fusion technology had been lost. Following rediscovery, the Graysons widely adopted fusion, including its use in hyper-capable warships such as GNS Saul and Covington. However, the pre-alliance Grayson fusion plants were four times as massive as modern reactors of similar output and fissionable materials were plentiful, so fission technology was not abandoned and actually turned out to be useful in developing several new classes of warships. When the Manticoran Alliance introduced its new LACs at the end of the First Havenite-Manticoran War, they were powered by state-of-the-art fission plants. (HH8)