A crossover is where aspects of two or more distinct fictional universes are included within the same story.
Crossovers[]
Star Trek universe[]
Episode 3, "Cybernetic Heartbeat", of Star Trek: Brave New Worlds involved a crossover with Star Trek: Pretoria. Both series were separate message board forum role playing games on the now defunct The Helix BBS in 2004. Each RPG runner, Hawku (at the time known as Aku) and Birds of Prey, merged each series' next "episode" to start as one thread where RPG player/writers from both series would write their character's experiences in a shared situation.
Star Trek: Unity is a crossover that saw several separate fan fiction series combine to deal with the Vendoth.
"Remembrance of the Fallen" is a crossover between the Masterverse and Bait and Switch, two distinct Star Trek Online fanfic universes.
With other franchises[]
"Its Hour Come Round at Last" featured a crossover between Star Trek and Harry Potter.
"A Thunder to Shake the Stars" is a crossover between Star Trek and The Fairly OddParents.
The fan fiction Star Trek: Voyager - Dark Realm included crossovers with Back to the Future and Rocko's Modern Life.
Some early episodes of Star Trek: Phoenix-X crossed over with other fictions. "Fight, Parts I, II & III" crossed over with Xena: Warrior Princess. "Alien Interference" had a subtle crossover of the Alien (Alien franchise) into the Trek universe.