Recursive canon, as described in the TV Tropes website, is the use of self-referential fictional lore (usually in the form of merchandise related to the lore) within the setting of another related fictional world. A classic example is from DC Comics, where the Silver Age version of the Flash, Barry Allen, reads about the exploits of the Golden Age version, Jay Garrick, in a comic book, and then later travels to an alternate universe where Jay Garrick is an actual person and the stories he has read about him had actually happened in Jay Garrick's universe, which was later named Earth-2. (The Justice League two-part episode "Legends" also plays along those lines.) A more recent example is in the movie TRON: Legacy, where the TRON videogame created by Bally-Midway in the real world was actually developed in-universe by Encom in order for Kevin Flynn to cash in on his experiences in the computer world in the movie TRON.
The 2011 Smurfs movie uses recursive canon in that the Smurfs comic strips were actually renditions of a historical legend documented by Peyo, who in the real world is their creator. The in-universe discovery date of 1958, as seen in the Google search engine result page, is also the same year that the Smurfs first appeared in the comic book story "The Smurfs And The Magic Flute" (as part of the Johan And Peewit comic book series) in the real world.
- Recursive canon at TV Tropes