Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. An example is two hydrogen nuclei joining together to make a nucleus of helium.
In this process some of the mass may be converted into the energy of radiation. The new nucleus therefore has a little less mass than the two nuclei that joined to make it, and the difference is released as energy.
Fusion is the opposite of fission. Fission is the splitting of a large nucleus into smaller ones, whereas fusion is the joining of small nuclei into a larger one.
