Web fiction
Shared universe
A fictional universe shared by a set of distinct creative works that each stand alone while jointly developing the same world.
A shared universe (or shared world) is "a fictional universe from a set of creative works where one or more writers (or other artists)" contribute works that stand alone yet "fit into the joint development of the storyline, characters, or world of the overall project," a setup the article calls common in science fiction. It differs from collaborative writing, several creators working on one work, and "from crossovers where the works and characters are independent except for a single meeting." The examples span media: the Cthulhu Mythos expanded by August Derleth, Larry Niven's Known Space opened to other writers, Thieves' World, the Marvel Universe, and television's Whoniverse of Doctor Who spin-offs.
Consistency is the shared universe's standing cost: with many hands, facts drift, and the article names canon and retconning as the tools that manage it. For writers the discipline is explicit worldbuilding: rules written down, plus canon decisions when contributors disagree. The one-off meeting of separate worlds, by contrast, is a crossover, not a shared universe.