From Champions MUSH

 This article has not yet been officially approved.

Roughly speaking, a dimension defines a set of physical laws. Fundamentals like gravity, time, space, thought, energy, mass, etc. might work differently, in minor or major ways, in different dimensions.

Other dimensions are variously referred to as "planes" or "realms" or "universes" or "worlds" or "realities" or various other terms. There are technical differences of interest only to an expert; for practical purposes they are all interchangable.

Dimensions can be broadly divided into three categories:

• "Ordered" dimensions have consistent and predictable physical laws that control events. Ordered dimensions with a common physical structure but different history are sometimes called alternate timelines.
• "Chaotic" dimensions have no such predictability. Events occur randomly, or in response to the thoughts of people in it. Some examples include the Elemental planes and Dream dimension.
• "Divine" dimensions are the home planes of godlike entities. Events, laws, etc. reflect the wishes of those entities and nothing else. They may resemble ordered or chaotic dimensions, depending on the nature of the entities who live there. Some examples include the Totem Spirit plane.

Almost all site activity takes place in an ordered dimension sometimes known as the "prime material plane" (among mages) or Universe-1 (among scientists). This is the home dimension for pretty much all PCs and NPCs and it behaves roughly the way the real world does (at least, to the extent that 5ed rules reflect the real world accurately).

Most magic involves interfacing with other dimensions, allowing the physical laws of the material plane to be bypassed. In almost all cases the interface is with a chaotic dimension. However, not all such interfacing is magic.

Many forms of Teleport and Summon, and more generally powers that involve causing mass or energy to appear literally out of nowhere, or to move from one place to another without occupying intervening space, probably involves interdimensional interfacing. (This is really an SFX question... a fireblast might draw from the Fire Dimension, but is usually just a local energetic phenomenon.)

Travelling to other dimensions requires Extradimensional Movement (XDM). PCs can buy this power, though GMs will review it carefully. You can have such travel in your background even if you don't have the power.

Subplanes

Local conditions within a dimension can vary radically, just like the surface of Earth differs from the surface of Jupiter.

For example, Midnight and Regent both obtained some of their powers through interfacing with the Darkness dimension. However, when they perceived or entered their Darkness dimension it appeared and behaved very differently, because they were in different parts, or "subplanes," of it.

Travel between subplanes within a dimension may be as simple as walking to a nearby town or as difficult as getting to another planet, depending on the specific case, but does not generally require XDM.

Some known subplanes:

• Atlantean Elemental Realms - Planes of Earth, Air, Fire, Water, Light and Dark, each part of the corresponding Elemental planes. They are all tightly interconnected with one another through natural portals. Each is governed by a central authority of some sort -- king, or whatever -- and they have active (though not necessarily friendly) relations with one another. Were actively used by the ancient Atlanteans and now used by Regent. New Blood had a series of adventures there.
• Darkplane - Once the source of Midnight's power, now primarily accessed by Dr. Lawrence Furmano. Part of the Elemental planes of Darkness. Uninhabited, featureless black void, the substance of which responds very actively to human thought.
• Infernal Realms - These are subsets of Hell. There are lots of them.
• Thunder and Lightning Realms - Part of the Elemental planes of Air.
• Plant realm - Part of the Elemental planes of Earth.
• The Beetle Demesne - A very large region in the Totem Spirits plane.