Analysis / Fusion Fic

Difference between Fusion Fic and traditional Crossover

When most people think of a "Crossover", they typically think of one of the following:

The common thread here, is that for some reason or another you have one work's characters showing up alongside another work's characters. The crossover can be independent of both sources, or one source can be the host while the other is the guest, or it can be mutual, but that's usually what a crossover means.

Fusion Fic is distinct from other crossover types, in that it blends the very premises of the works rather than merely putting their characters together. Either it puts the characters from work A into work B, but with A's characters replacing rather than appearing alongside B's, or it blends together two distinctive worlds, or both.

There's also sort of a lightweight intermediate form between Fusion Fic and traditional Intercontinuity Crossover, where work A keeps its setting and premise 100% intact (aka "canon compliant" in fanfic terminology), while guest characters from work B have their backstories altered (sometimes massively) to be compatible.
http://tvtropes.org/pmwiki/pmwiki.php/Analysis/FusionFic