Nuclear Fusion
Definition
Joining light nuclei into heavier ones, releasing energy: the process powering the Sun and stars.
Technical definition
Nuclear fusion is the merging of light nuclei, such as hydrogen isotopes, into heavier ones, releasing energy from the mass defect when the product is more tightly bound. It requires extreme temperatures and pressures to overcome electrostatic repulsion, as in stellar cores.
Example
The Sun fuses 600 million tons of hydrogen every second, converting about 4 million tons of mass directly into energy.
Common confusion
Fusion is not clean fission. It needs temperatures hotter than the Sun's core to start, which is why controlled fusion power remains an unsolved engineering challenge despite decades of work.