Fusion (archetype)
"Fusion" (フュージョン Fyūjon) is an archetype of cards mainly used to Fusion Summon Fusion Monsters. It is closely tied to the "Polymerization" archetype.
While many cards with "Fusion" in their names are actually in the "Polymerization" archetype instead (or are in both despite only having "Fusion" in their name), all support cards for this archetype also support the "Polymerization" archetype at the same time, making this a non-issue. "Diffusion Wave-Motion" is not a "Fusion" nor a "Polymerization" card due to its Japanese name, so it includes an archetype-exclusion condition to indicate this.
Not all cards that can Fusion Summon include "Fusion" or "Polymerization" in their names, and not all "Fusion" cards can Fusion Summon.
See also: Fusion Summon and Polymerization (archetype)