The combining of two small nuclei to produce heat and one larger nucleus is called nuclear fusion.

Here's a breakdown:

* Fusion: The process of combining two atomic nuclei to form a heavier nucleus.

* Small Nuclei: Typically isotopes of hydrogen (deuterium and tritium) are used.

* Heat: A tremendous amount of energy is released in the form of heat and light.

* Larger Nucleus: The resulting nucleus is heavier than the original two nuclei, with some of its mass converted into energy according to Einstein's famous equation E=mc².

Nuclear fusion is the process that powers the sun and other stars. It is also the subject of intense research as a potential source of clean and abundant energy for the future.