Pocket dimension
Pocket dimensions are confined "pockets" of physically-condensed space found within other, more established dimensions. Essentially, pocket dimensions are isolated "sub-dimensions" found within others. Although they are geographically separate from their "host" dimension, and thus can only be accessed through teleportation or similar magical methods, they are actually part of the host dimension on a more metaphysically complex level.
Pocket dimensions are referred to by various characters, although some of the most obvious examples of this are in the Runecrafting Guild, where Wizard Elriss and similar scholars are studying them, amongst other subjects. The Runecrafting Guild itself is found in a pocket dimension that is apparently located in the "shadow" of the original Wizards' Tower. Another example are the runecrafting altars found throughout Gielinor. The altars are pocket dimensions said to resonate with their respective energies; for instance, the air altar resonates with the most air magic of any location in Gielinor.
Apparently pocket dimensions can be "flattened" and physically brought into their host world by extremely powerful mages. According to the Oneiromancer found on Lunar Isle, the astral altar was physically brought into Gielinor by the Moon Clan.