In RuneScape, "Worlds" are universes parallel to one another. These include not only the world of Gielinor, but also our own, and those in other fictional works.
“The Dream World is the realm that players are required to go to in the quests: Lunar Diplomacy...Dream World
“Puro-Puro is a magical world, separate from Gielinor, and is the main location for the Impetuous...Puro-Puro
“The Fisher Realm is world parallel to Gielinor, a small part of which can be entered by blowing...Fisher Realm
“Yu'biusk is the home plane of the Goblins, ogres, orks, and ourgs, separate from the RuneScape...Yu'biusk
“The Spirit Plane is a plane of existence from which Summoning energy can be used to summon...Spirit Plane
“The Shadow Realm is a mysterious location in Gielinor. Although it seems to be part of the world...Shadow Realm
“The Spirit Realm is a different plane, parallel to the Wilderness. It is accessible during and...Spirit Realm