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A crossover is when two or more normally discrete settings are combined in a single piece of media.
The most common example are characters from unrelated stories appearing together. One character cosplaying as a character from another series does not normally qualify as a crossover, both settings have to be present in a "real" way. Instances where a piece of media with a single character would qualify as a crossover would be a character from one setting in a recognizable location from another setting, or using abilities or objects that come from a specific and recognizable different setting.
When things from the same franchise that normally remain entirely discrete meet, such as two characters from different games that use the same setting, use in-franchise crossover instead.
Crossover should not be confused with parody. A parody implies mimicking a scene or dialogue from another series, with the intention of being humorous.
Use songover for crossovers between multiple songs or other musical compositions, seen commonly with Vocaloid songs.
See also
The following tags implicate this tag: multiple_crossover (learn more).