Worldbuilding is the process of constructing an imaginary world, sometimes associated with a whole fictional universe. The term "world-building" was first used in the Edinburgh Review in December 1820 and appeared in A.S. Eddington's Space Time and Gravitation: An Outline of the General Relativity Theory (1920) to describe the thinking out of hypothetical worlds with different physical laws. The term has been used in science fiction and fantasy criticism since appearing in R.A. Lupoff’s Edgar Rice Burroughs: Master of Adventure (1965). The resulting world may be called a constructed world. Developing an imaginary setting with coherent qualities such as a history, geography, and ecology is a key task for many science fiction or fantasy writers. Worldbuilding often involves the creation of maps, a backstory, and people for the world. Constructed worlds can enrich the backstory and history of fictional works, and it is not uncommon for authors to revise their constructed worlds while completing its associated work. Constructed worlds can be created for personal amusement and mental exercise, or for specific creative endeavors such as novels, video games, or role-playing games.
The creation of literary fictional worlds was first examined by fantasy authors such as George MacDonald, J. R. R. Tolkien, Dorothy L. Sayers, and C. S. Lewis. Tolkien addressed the issue in his essay On Fairy Stories, where he stated that the "Secondary World" or "Sub-Creation" (the constructed world) is substantially different from the art of play-writing: "Very little about trees as trees can be got into a play." Constructed worlds shift away from storytelling, narrative, characters and figures, and may explore "trees as trees" or aspects of the world in-and-of-themselves. Later analysis of fantasy worlds in the 1960s contextualized them in the medium or the narrative of the works, offering an analysis of the stories in the world, but not the world itself. Media franchises are similar to constructed worlds but do not necessarily rely on them; for example a media franchise such as Star Wars has a constructed world, while The Beatles, as a franchise, does not. In the 2000s, world-building in film has increased in popularity. When before, writers sought to create a character that could sustain multiple stories, now they create a world that can sustain multiple characters and stories.