The word "fusion" refers to the merging of two or more distinct elements into a single whole. It's a pretty universal concept, with a great many new genre and style coming about due to thinking X would taste great with Y.
Tropes involving fusion:
- Combining Mecha, when two or more Mecha have the ability to combine with each other into a new one.
- Composite Character, which is when a writer combines two or more character into a single one in the process of adaptating a work.
- Fusion Dance, which is when two characters fuse into a single being.
- Fusion Fic, a type of Crossover fanfic where the characters of one or more franchises replace the established characters in yet another franchise.
- Ninja Pirate Zombie Robot, which is when multiple cool things are combined for the sake of making something even cooler, often in logically impossible or contradictory ways.
- Patched Together From The Headlines, when a story is a fusion of multiple high-profile news stories.
- X Meets Y, a way for describing a product by expressing it as the sum of two separate unrelated products.