Nuclear fusion

Nuclear Fusion is the process whereby a number of smaller atomic particles are fused into a large atomic particle, releasing a considerable amount of energy in the process. Fusion occurs naturally inside stars, and many races have been able to create artificial fusion reactions to provide energy for their own use.

For a number of years, humans were unable to create a sustained fusion reaction. As a result, humans used nuclear fission quite extensively during the 20th and 21st centuries. Humans were later able to create a sustained artificial fusion reaction, leading to the replacement of the older fission reactors. Fusion reactors became an everyday part of life in the United Federation of Planets. (Star Trek IV: The Voyage Home).

On board space vessels, in addition to providing power for general use, nuclear fusion reactors are also used to provide energy for a ship's impulse drive. On Federation starships, reactors are modular devices, which are moveable and can be brought into position depending on the energy needs of a starship. (Reference work Star Trek: The Next Generation Technical Manual).

By the 24th century, fusion reactors had become small enough that many homes had their own fusion reactors to provide power for their occupants. The house of Kevin and Rishon Uxbridge had a small fusion reactor, that Commander William T. Riker in 2366 estimated would last another five years. (TNG episode The Survivors).