Etymologically, a hub is the central part of a wheel, which connects all the spokes. The metaphor is often used for anything that connects together a large number of identical things. In ZDoom, a hub is technically an entire cluster of levels. The term is also sometimes used in a way closer to the original image, by qualifying one level in particular to be the hub for a cluster, for example the Seven Portals map of Hexen is the map from which all other maps in the cluster can be reached.
To define a hub, at least two maps need to be in a single cluster, which needs to have the hub keyword in its cluster definition. Many features work differently depending on whether the current level is part of a hub or part of a "normal" cluster of sequential levels.
- ZDoom saves the status of each visited level in the cluster. In a non-hub cluster, revisiting a level resets it.
- There is no intermission screen with kill/item/secret tallies between hub levels.
- The inventory remains unchanged when traveling between hub levels; except for Powerup items without the INVENTORY.HUBPOWER flag.
- By default, the automap does not show the map label when in hub maps.