crossover

A crossover is a story bringing characters from separate works together. A crossover episode, a comic crossover, the crossover everybody wanted.

It is standard vocabulary in television, comics and games, where whole franchises are built on them. The word also means an artist moving from one audience to another — a crossover hit is a song that succeeds outside its own genre. In cars, a crossover is a body style between an estate and a four-wheel drive. Note crossover as an adjective, and cross over as two words, which is simply the verb.