A crossover is when two or more characters from unrelated copyrights are shown together in one 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.
One character cosplaying as a character from another series does not normally qualify as a crossover.
Use songover for crossovers between multiple songs or other musical compositions, seen commonly with Vocaloid songs.
The following tags implicate this tag: multiple_crossover (learn more).