Multiverses in sci-fi and fantasy: naming logic
Dimensions function narratively as characters: they need evocative names, internal logic and a clear position in the world's cosmology. D&D's Astral Plane, Final Fantasy's The Void, Stranger Things' Mirror World: each communicates nature through name before explaining rules.
The best dimension names combine spatial category (plane, sphere, realm) with sensory quality (ethereal, burning, whispering). This combination gives immediate sense of what being there feels like. The Whispering Realm suggests something audible and discreet; The Burning Abyss suggests literal hell. Word choice saves you paragraphs of description.
In complex multiverses (Marvel Comics, Magic the Gathering), dimensions have short codes for character internal conversation: Earth-616, Ravnica, Innistrad. This works because there are dozens of worlds and expert readers recognize them. For initial worldbuilding, avoid this: until you have at least 5 relevant worlds, codes sound pretentious.
Typical plane categories in RPGs
D&D codified a taxonomy many systems inherited. Material Plane is the world where action happens. Elemental Planes (Fire, Air, Water, Earth) are primordial sources. Outer Planes (Heavens, Hells, Abysses) have moral alignment. Transitive Planes (Astral, Ethereal) are corridors between dimensions. Your cosmology can break this taxonomy but worth knowing because players bring it as expectation.
If you want something different from the D&D formula, look at real or literary cosmologies: Norse Yggdrasil has 9 worlds connected by branches; Hindu cosmos has 14 lokas; Gaiman's The Sandman has Dream, Waking, Mirror, Beyond. Any of these templates gives you structure without feeling derivative.
For sci-fi, dimensions tend to be parallel universes with alternate physical laws, not mystical planes. Slow Universe where light travels at 60 km/h, Carbon-Hostile Universe where silicon-based life is norm. Scientific speculation replaces mysticism: physical rules change, not just atmosphere.
Common mistakes when designing dimensions
Dimension inflation: if your world has 47 different planes, none has narrative weight. Memorable fictional multiverses have 5-12 relevant dimensions with clear personality. Beyond that, it becomes manual listing.
Dimensions that are just 'the normal world but worse': if your Mirror World is exactly like reality only darker, it's not interesting dimension. Each plane should break something fundamental: physics rules, time's arrow, consciousness nature, causality. Stranger Things does well adding permanent climate and hostile life to the Upside Down; would be boring if just dark Hawkins.
Lack of clear transition mechanisms: if characters can jump between dimensions at will without rules, you lose narrative tension. Define: how to enter? what does it cost? are there risks? Marvel uses wands, spells, machines; Sliders used a device with forced cooldown. Friction to cross dimensions is what makes them special.
Finally, dimensions that break your own worldbuilding: if you establish that the dead go to the Gray Underworld and then introduce the Realm of Oblivion where souls dissolve, readers get confused. Maintain metaphysical coherence: each plane must have unique function in your cosmology.
How to give depth to a named dimension
After choosing the name, define five traits so the dimension feels alive. Sensory atmosphere: what looks, sounds, smells, feels different? The Astral Plane smells like static, the Broken Veil tastes like metal and honey.
Inhabitants: who or what lives there? Can be native, refugees from other dimensions, or entities only existing in that plane. D&D's Slaadi only exist in Chaotic Limbo; that justifies why Limbo is different.
Internal geography: does the dimension have its own map? Even conceptually, define 3-5 notable locations. The Dream Realm has the House of Mystery, the Cemetery of Forgotten Dreams, etc.
Alternate rules: what works differently? Slower time, inverted gravity, amplified magic, temporary death. Each broken rule must have narrative implication: if time passes slower, characters can return to the material world and discover 100 years passed. That generates plot.
Prior history: what happened there before? Planes without past feel like sets; with past they feel like real places. An ancient cosmic war, an imprisoned god, a collapsed civilization: any event lends weight to the plane without needing to narrate extensively.