A fusion reactor was a type of power-generation devise using nuclear fusion of hydrogen isotopes into helium. Fusion reactors were commonly employed to generate power in the starships and facilities of many races.
Fusion reactors were used to power the impulse engines on Federation starships. In a Galaxy-class each impulse engine had three impulse fusion reactors to provide the power. (TNG reference: Technical Manual)
Fusion reactors were also used to provide power for large facilities. The Delphic Expanse spheres were powered by seven twelve kilometer long fusion reactors. Cardassian Nor-class space stations also used fusion reactors. The fusion core of a Nor-class facility held six fusion reactors, which were more than enough to power the entire station. (ENT episode: "Anomaly"; DS9 reference: Technical Manual)
In April of 2376 the Jem'Hadar Kitana'klan attempted to destroy DS9 by initiating an overload in the station's fusion reactors. He was successful in starting the overload, but fortunately station personnel were able to jettison the fusion core before it exploded, saving the station. (DS9 novels: Avatar, Book One, Avatar, Book Two)