A crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, or because of unauthorized efforts by fans. Generally, unofficial crossovers take the form of fan-written fiction and fan art, but it is increasingly prevalent in ameteur films and audio.
Types of crossovers Edit
A crossover is when the characters meet each other, still retaining all the elements that make the characters what they are. For example, see CaptainKodak's The Muppet Show with their guests Team Possible.
A fusion is when you blend the characters into a particular universe, meshing characteristics of both genres to produce something unique, and yet similar to each. For example, see Classic Cowboy's The Batman.
An AU is just a story that ignores a portion or all of canon in the telling of it's tale to achieve a desired purpose. For example, see MrDrP's Nacho Boy and the Dragon Lady.