Fusion

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Fusion is the process of two or more headmates or fragments becoming one. The resulting headmate (sometimes referred to as a "fusion" themself) often has traits of both, but not always balanced.

In large systems, especially polyfragmented ones, fusion can be common. Fragments often coalesce by accident based on shared traits, for example.

In some circles, fusion can also be non-permanent; some theorize the popularity of Steven Universe had an influence on the use of fusion in that way, or it could be attributed to the fluidness of plural terminology. Either way, some systems may use "fusion" like merging or blending.

History
Some systems force members to fuse together without one or both member's consent. This is considered inhumane and is often the result of pressure from external sources, especially doctors or fellow systems. To the rest of the system, a sudden fusion can feel like the sudden death of a family member or friend.

Fusing is not inherently bad or good, and can result in increased stability for the whole system; in the end, though, the choice to pursue large or small scale fusions is often a private matter and is up to every individual system.

Related Terms
A non-permanent merging of headmates is referred to as blending or merging, which can sometimes be a precursor to a fusion, and fusiotien systems in which the majority of headmates are fusions.

Fusion is also the last step in the process of integration, or the lowering of barriers between headmates; the lowest the barriers can be is non-existent, after all. This can also end in final fusion, or when all headmates merge together, leaving one headmate that is the sum of all.