Nuclear fusion is a physical reaction that can be harnessed as a form of power generation; it is the very process which causes stars to shine.
In fusion, two lighter elements are combined under intense heat and pressure. The two elements fuse together, forming a single, heavier element. This reaction generates a significant amount of energy, the exact amount depending on the elements used and the resultant element created.
One of the most efficient and powerful forms of fusion is hydrogen fusion, whereby two atoms of deuterium are fused into helium. This reaction is used in fusion reactors and is also found in early to older stars.
Nuclear fusion reactors were frequently used on starships as impulse engine power plants and auxiliary power generators. (citation needed • edit)
Helium fusion enhancement was at the center of Doctor Timicin's studies to re-vitalize the Kaelon sun. (TNG: "Half a Life")