Between all things flows the Ethereal. It is the medium that separates and connects all places. Every plane of existence is surrounded by and connected through the Ethereal. When a wizard teleports, they briefly dissolve into the Ethereal and reconstitute elsewhere. When a cleric's prayer travels to their god, it passes through the Ethereal. When a soul departs a body, it drifts through the Ethereal. Magic itself flows through the Ethereal like blood through veins.
The Ethereal does not move. The Ethereal is movement.
What the Ethereal Looks Like
Near the Material Realm, travelers see ghostly echoes of the Material world - faint impressions of buildings, roads, and forests built from silver mist. These are resonance impressions, the Material world's presence imprinted on the adjacent Ethereal like heat radiating from warm stone.
Deeper into the Ethereal, the echoes fade. Thoughts have physical form here - a strong enough emotion can manifest as weather, terrain, or architecture. The ruins of civilizations that have forgotten themselves drift through the deep Ethereal as memory-fossils, perfectly preserved but inhabited only by crystallized recollection.
Thought as Physical Substance
In the Ethereal, thought is the physical substance other things are built from. A strong enough fear, held by enough people, can calcify into something you could walk around. Travelers carrying strong unresolved emotions may find those emotions taking form around them. Not as hallucinations. As participants.
The Astral Layer
High in the Ethereal, where distance ceases to be a physical property and becomes a matter of intent, lies the Astral. Gravity loses meaning. A traveler moving astrally does not walk - they aim their awareness and arrive. The Astral appears as a vast sea of slow silver stars, or a current of pure thought connecting all realms simultaneously. Both descriptions are correct. The Astral is the Ethereal at its most transparent, when the medium reveals its fundamental nature.
Magic's Highway
Every spell that has ever been cast in the Material has passed through the Ethereal. When a wizard conjures fire, the spell-form exists simultaneously in Material and Ethereal. Divine magic travels differently - a cleric's prayer passes through the Ethereal like a thread of intention seeking resonance, moving not in a straight line but along the paths of strongest connection between caster and deity.