Spirit World

The Spirit World is a parallel realm of existence that spans across the XSI multiverse. It is highly empathic, and even the non-sentient matter will commonly react to the thoughts and emotions of the people within. The primary occupants are almost all Spirits.

Mortals can travel to the Spirit World through several means. Sorcery allows one to access the Astral Dimension, a specific Earths-adjacent plane of the Spirit World. Spiritually connected, knowledgeable, or enlightened people can even transport themselves to almost any part of the Spirit World. Some Earths have Spirit Portals that can be used to physically travel to the Spirit World.

The landscape of the Spirit World is not spatially consistent. It moves and changes as the minds of people influence it. However, there do exist "fixed" locations, such as Shazam's tower or Earth 21's Spirit Portals. These locations don't change as much, because over time they have hardened to a near-physical state due to consistent conceptual perception of them. But their placement in the surrounding world is not as stable.

Spirits
Spirits are a set of species of entities that exist without physical form. Spirits can be personified manifestations of a location, a concept, or even a strong emotion. Certain mortals who have an established connection with the Spirit World can become Spirits themselves, and go to the Spirit World either before or when they die.

Spirits can have vastly distinct forms and abilities. Most can turn intangible or invisible at will, even when in a physical realm. Some can transport to and from one or more Earths at any time. Some can possess the physical forms of mortals, which may or may not cause deformities afterward. Others, as personifications of a thing can to varying degrees manipulate that thing.

Locations
Shazam's tower

The home made by the wizard Shazam after his departure from the physical realm. A large non-euclidean tower that contains many powerful artifacts, including a Makluan Ring.

Astral dimension

The closest part of the Spirit World to any given physical universe. Here, the Spirit exists within and can observe the spatial universe without physical form or intentional interaction.

Iroh's Spirit tea shop

The Fog of Lost Souls

A living Spirit that smothers all others that enter its canyon, making them forget all but their darkest thoughts and memories and trapping them with their own paranoia.

Earth 21's Spirit Portals

Three portals that can be used to freely and physically travel between the Spirit World and Earth 21.

Occupants
Zhi Hou

Shazam

Iroh

Avatar Korra

Koh the Face Stealer

Unnamed Ten Rings Spirit

See Also:
Spirit World, on Avatar Wiki

Astral Dimension, on Marvel Cinematic Universe Wiki

Haunted (episode), on Arrowverse Wiki