In nuclear fusion, two or more small and light nuclei come together to form a heavier nucleus.
This process is accompanied by the release of a huge amount of energy.
Below are two examples of fusion reactions:
Fusion is much more difficult to achieve than fission because the hydrogen nuclei repel each other. Therefore, the nuclei must be heated to 108 K or more so that the nuclei will have enough of kinetic energy to overcome the electrical repulsion between the nuclei.
Example
The sun get its energy from the fusion of hydrogen nuclei.
A hydrogen bomb uses the principle of nuclear fusion for its design.