The game world, also called the fiction, is the imagined space that is shared by all the players of a group and within which all the fictional events of a role-playing game take place. Events in the game world (or "in the fiction") occur by consensus of the players and according to the system they are using to play (per the Lumpley Principle).
The game world is distinct from a game's setting. The game world emerges from gameplay, but the setting includes information that informs gameplay (that is, to the extent that setting information is understood and agreed by the group, it is part of that group's system). Because the game world emerges from gameplay, it is limited to what is experienced and discussed during a role-playing game (including, potentially, bluebooking or other activities between sessions), but the setting may have a broad scope beyond that, particularly if it is from a published game or product. However, once information is established in the game world, it will become part of the setting of that ongoing game (in line with the causality clause of the narrative principle).