In Onaria - the Great Dream - there are the Three Planes of Existence, also known as the Planes of Onaria or Onarian Domains. Each Plane has Realms, also known as "planets", within, and are important parts of Onaria. Onaria cannot exist unless all three Planes are present.
Caelius
Caelius is the Immortal Plane, also known as the Domain of the Gods. It is outside of time and is noncorporeal. Thus, Caelius is limited neither by the temporal nor by the material. It is hard, if not impossible, for the mortal mind to comprehend Caelius in its true state, so when a mortal visits Caelius, they usually are given a false vision so that they are not eternally scarred by the magnificence of Caelius. Those few mortals who have seen Caelius' true form and survived have gone completely mad.
Mundus
Mundus is the Mortal Plane, also known as the Domain of Mortals. It is limited temporally and physically, bound by scientific law although bent by the one connection Mundus has to Caelius - Magick. What humans tend to call "space" is known as the Mundiun Nihilus, the Great Void of Mundus in which the Mortal Realms were created. The first Mortal Realms were Glacia and Ignia - a planet of ice, and a planet of fire, both created by Imium and Audulium after time began to tick in Mundus.
Arborius
Arborius is the Great Gateway. Arborius is known as the Tree of the Cosmos, or the Cosmic Tree. It interconnects all the realms of Mundus and Caelius, allowing cosmic gateways to be opened between them. It is, however, extremely hard to open a gateway from Mundus into Caelius. Caelans find it easy to accomplish this task, but Mundiuns do not. However, opening gates within Mundus, from one Mundiun Realm to another, is not anywhere near as hard. The branches of Arborius, known as the Pontes, allow seemingly near-instantaneous travel between the realms. Arborius transcends both Mundus and Caelius, therefore time is odd there. The Pontes form an intricate plane which is separate from yet interconnects to Mundus and Caelius. Oftentimes, one does not remember being within the Pontes, and thus it seems as if the transfer from one Realm to another is instantaneous, when in reality, it is not. One can also exit the Pontes before arriving at a realm, but this can be dangerous.