What process does 'fusion' refer to in contrast to another nuclear process?

Fusion refers to the process in which two light atomic nuclei combine to form a heavier nucleus, accompanied by the release of a significant amount of energy. This is the reaction that powers stars, including our sun, through the fusion of hydrogen atoms into helium.

In contrast, the other processes listed do not represent the same type of nuclear reaction as fusion. For instance, fission is a nuclear reaction where a heavy nucleus splits into two or more smaller nuclei, along with the release of energy. This is fundamentally different as it involves the splitting rather than the combining of nuclei. Variation, separation, and combustion are not related to nuclear reactions in the way that fusion and fission are. Therefore, fission contrasts with fusion because it involves splitting rather than merging nuclei, thereby making it a key point of differentiation in the study of nuclear processes.