A microfusion reactor was a power generation device utilized by several civilizations.
In 2373, Crewmen Pechetti and Stolzoff were tasked with bringing Empok Nor's microfusion reactor, located in the habitat ring, back online in order to assist in boosting power to the station's induction grid. (DS9: "Empok Nor")
The USS Defiant was also equipped with microfusion generators. Prior to departing Deep Space 9 in 2375, bringing them online was part of the impulse pre-start sequence. (DS9: "The Changing Face of Evil")