When characters from unrelated franchises are shown together in the same scene.
Crossover should not be confused with parody. A parody implies mimicking a scene or dialogue from another series, with the intention of being humorous, while a crossover simply implies two characters from different franchises meeting. A character cosplaying as a character from another series also does not normally qualify as a crossover.
See songover for crossovers between multiple songs or other musical compositions, seen commonly with Vocaloid songs.
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