A pocket world is a realm in the Ethereal Plane connected to a planet in the Material Plane. Because the construction of a Nexus requires both high mana requirements and a stable position in the universe, they can only be constructed inside of pocket worlds.
Known pocket worlds include Torn Elkandu, Drakanna, and Calarey. None of them are currently functional at present. Drakanna and Calarey were destroyed during the Elkandu Crisis, and Torn Elkandu was shut down when Falk came into power.