Here's why:

* Fusion: This term refers to the combining of two or more things. In nuclear fusion, we're talking about combining atomic nuclei.

* Nuclear: This signifies that the process involves the nucleus of an atom, which contains protons and neutrons.

* Low Mass: Fusion typically involves lighter elements, such as hydrogen isotopes (deuterium and tritium).

* Higher Mass: The fusion process results in the formation of a heavier element, like helium, while releasing a tremendous amount of energy.

Key Example: The Sun's energy is produced through nuclear fusion, where hydrogen nuclei fuse to form helium.