The energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity . In these
processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m is mass, and c is the speed of light.
Here is a good powerpoint on it
http://www-star.st-and.ac.uk/~kw25/teaching/as2001/sse/sse_5.pdf