Final answer:
Nuclear fusion is the process in which lighter elements combine to form a heavier element, releasing a significant amount of energy. It is the source of energy in stars.
Step-by-step explanation:
Nuclear fusion is the process in which two or more lighter elements combine to form a single heavier element. It releases a significant amount of energy, and it is the source of energy in stars. For example, in lighter stars, hydrogen combines to form helium through the proton-proton chain. The fusion reaction can be represented by 2H + H → ³He + energy.