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Nuclear fusion is a process in which two or more lighter atomic nuclei combine, or "fuse," to form a heavier nucleus. This process typically occurs at extremely high temperatures and pressures, resulting in the release of a tremendous amount of energy in the form of light and heat.

The energy released during a nuclear fusion reaction is a result of the mass defect, which is the difference between the combined mass of the initial atomic nuclei and the mass of the final nucleus formed. According to Einstein's famous equation, E=mc^2, this mass defect can be directly converted into energy, where E is the energy released, m is the mass defect, and c is the speed of light. Nuclear fusion reactions typically release a large amount of energy due to the strong nuclear force that binds nucleons (protons and neutrons) together in the nucleus.

Nuclear fission is a process in which a heavy atomic nucleus splits into two or more lighter nuclei. This process also releases energy in the form of light and heat, as well as neutrons that can induce further fission reactions in a chain reaction. Fission is the process used in nuclear power plants and atomic bombs.

Similar to nuclear fusion, the energy released during a nuclear fission reaction is also a result of the mass defect between the initial heavy nucleus and the lighter nuclei formed. However, due to differences in the nuclear binding energies of the nuclei involved, the energy released per unit of mass in nuclear fission is typically lower than that released in nuclear fusion.

When comparing the energy released by nuclear fusion and nuclear fission, it can be seen that fusion reactions release a higher amount of energy per unit of mass than fission reactions. While the exact values depend on the specific isotopes involved, it can be generally said that fusion releases more energy per nucleon compared to fission. For example, the fusion of hydrogen nuclei to form helium releases about 7 MeV (Mega electron volts) of energy per nucleon, whereas the fission of uranium-235 releases about 200 MeV per fission event – which amounts to approximately 0.8 MeV per nucleon considering an average of around 236 nucleons in uranium-235 fission fragments. In summary, nuclear fusion reactions release more energy per unit of mass compared to nuclear fission reactions.