ARG or Alternate Reality Game
From Wikipedia:
(https://en.wikipedia.org/wiki/Alternate_reality_game)
An alternate reality game (ARG) is an interactive networked narrative that uses the real world as a platform and employs transmedia storytelling to deliver a story that may be altered by players' ideas or actions.
The form is defined by intense player involvement with a story that takes place in real time and evolves according to players' responses. Subsequently, it is shaped by characters that are actively controlled by the game's designers, as opposed to being controlled by an algorithm as in a computer or console video game. Players interact directly with characters in the game, solve plot-based challenges and puzzles, and collaborate as a community to analyze the story and coordinate real-life and online activities. ARGs generally use multimedia, such as telephones, email, and mail but rely on the Internet as the central binding medium.
from unfiction.com
https://www.unfiction.com/history/
Alternate Reality Gaming (also known as beasting, unfiction, or immersive fiction) is an interactive fusion of creative writing, puzzle-solving, and team-building, with a dose of role playing thrown in. It utilizes several forms of media in order to pass clues to the players, who solve puzzles in order to win pieces of the story being played out.
Clues can be passed through web pages, email, voicemail, snail mail, television advertisements, movie posters, campus billboards, newspaper classifieds… really, in any way that information can be passed.
Many times, the puzzles that must be solved cannot be solved alone. This genre of game almost requires participation in a group or community that works together to win past the more difficult hurdles.
Beasting is unique in that it wouldn't be possible without the community-building and networking power of the Internet. Besides the obvious fact that there would be no web pages or email in which to hide clues, it would be nearly impossible for the diverse audience to coordinate the sheer amount of data, speculation, and solutions among players.
Transmedia storytelling
From Wikipedia:
(https://en.wikipedia.org/wiki/Transmedia_storytelling)
Transmedia storytelling (also known as transmedia narrative or multiplatform storytelling) is the technique of telling a single story or story experience across multiple platforms and formats using current digital technologies.
The study of transmedia storytelling—a concept introduced by Henry Jenkins, author of the seminal book Convergence Culture—is an emerging subject. Because of the nature of new media and different platforms, varying authors have different understandings of it. Jenkins states the term "transmedia" means "across media" and may be applied to superficially similar, but different phenomena. In particular, the concept of "transmedia storytelling" should not to be confused with traditional cross-platform, "transmedia" media franchises,[1] or "media mixes".
Henry Jenkins - Transmedia storytelling 101
http://henryjenkins.org/blog/2007/03/transmedia_storytelling_101.html