A hub is a map which serves to connect many other maps together. The function of a hub map is analogous to the function of the overworld in a role-playing game. The overworld acts as an "in-between" area that connects individual dungeons together in a nonlinear fashion. A hub does the same thing, except with the levels of a WAD. The first Doom-engine game to use a hub was Hexen. Hexen used the hub system to create puzzles that required the player to travel back and forth between levels in order to solve them. Strife also used the hub system.
Doom hubs are difficult to create and implement and are therefore infrequently used in pure Doom or Heretic ports. SMMU implemented a hub system for Doom; however, it was removed or disabled in its successors such as Eternity Engine because of potential problems in that implementation. However some source ports (such as ZDoom) support hubs, allowing them to be created for either Doom or Heretic levels; and several prominent megawads designed for ZDoom have utilized them, including Team Future's RTC-3057 and TeamTNT's Daedalus: Alien Defense.