Nuclear Fusion : The process of combining of the nuclei of light atoms to form heavier nuclei with the release of energy is called Nuclear Fusion.
In this process, some mass disappears. The lost mass appear in the form of heat and light energy and due to enormous amount of heat more and more nuclei fuse.
Thus, A normal chain reaction is produced with the release of tremendous energy.
Hydrogen Bomb is prepared on this principle. This reaction takes place in the sun and other stars and is one of the important source of stellar energy.