A fusion reactor or fusion power plant was a power generation device, which used nuclear fusion to generate energetic plasma for various uses.
Inside such a reactor, two deuterium atoms were fused together to form a single helium atom. This reaction generated a massive amount of energy, which could be used to power various systems.
Impulse power of Federationstarships was generated by fusion reactors. These reactors were components of the fusion reaction systems. Aboard the Galaxy-class starships, the primary systems were located on decks 23 and 24. (TNG: "Contagion", display graphic)
Cardassian-built reactors were based on a series of carbon reaction chambers, which uses laser-induced fusion. According to the specifications for Cardassian fusion reactors, the operating efficiency was acceptable at as low as twenty percent peak efficiency. In 2369, ChiefO'Brien conducted a diagnostic of Deep Space 9's fusion power plant. The results were found to be thirteen percent below normal, well with Cardassian specifications, but not O'Brien's. To increase operating efficiency, O'Brien increased the deuterium flow to the carbon reaction chamber by five percent, while at the same time observing the exhaust fusion temperature. (DS9: "The Forsaken")