Crossover episode

A Crossover or Crossover episode is a story with characters from two or more different stories.

 Movie Crossovers 
Universal Studios, in the 1930s, made the movies Dracula, Frankenstein, The Wolf Man, and The Mummy. They then started making movies where the characters met each other. The Friday the Thirteenth movies also had a crossover with the Freddy Krueger movies. More recently the movie Spider-Man: No Way Home was a crossover between every Live action Spider-Man movie in the 21st Century. 

 Cartoons 
There was also a movie where The Jetsons meet The Flintstones. The Simpsons has had crossovers with The Critic, Futurama and Family Guy. The Jimmy Timmy Power Hour is a crossover between The Fairly OddParents and The Adventures of Jimmy Neutron: Boy Genius. There was also a crossover between Phineas and Ferb and Milo Murphy's Law.

 Books 
Dr. Isaac Asimov also wrote crossovers between his Foundation series and his I, Robot sequels. Brandon Sanderson also writes crossovers in his Cosmere books. Rick Riordan has also written crossovers between his different Mythology books.

 Comics 
Comic books have used crossovers for a long time and used them to create groups like The Justice League and The Avengers.