Crossover
From Super-wiki
Definition
A crossover is a story in which one universe melts with another universe, or characters from one universe cross over into another universe. An example for a Supernatural/X-Files crossover would be a story in which Sam and Dean help Fox Mulder and Dana Scully investigate a supernatural phenomenon.
Examples for Crossover Fiction
[dotfic]'s [Looking for Salt] - Supernatural/X-Files
[trystan830]'s [Timeshifter Series] - Supernatural/Firefly
Links
[sn_crossovers] - Crossover Community on LJ