A solar fusion eruption was an ejection of superfluid gas from the surface of a star. It could be caused by a local destabilization of the photosphere, creating an expanding pressure wave that resulted in a subsurface fusion reaction.
In early 2370, the USS Enterprise destroyed a Borg vessel with a solar fusion eruption, triggered by directing a highly energetic particle beam from its tractor beam emitter at a particular location on the star. The technique was devised by Ensign Taitt, who did her senior honors thesis on solar dynamics. (TNG: "Descent, Part II")