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Dimension referred to a self-contained universe.
Earth, while also the name of a planet, was a dimension like many others. Some dimensions were parallel universes to the Earth only with a different timeline, like the Wishverse; other worlds had different laws of physics and magic, like Quor'toth and Pylea.
The flow of time within other dimensions were often different from that on Earth. For example, Angel was trapped in Acathla's dimension for at least three months in Earth time, but was said to have endured hundreds of years of torture. Connor also returned from Quor'toth in his mid-to-late teens after having entered the dimension as an infant mere weeks beforehand in Earth time.
However, this did not seem to be the case with all dimensions; for example, Winifred Burkle was trapped within Pylea for five years in both Earth and Pylean time.
- “What is Hell but the total absence of hope? The substance, the tactile proof of despair.”
A hell dimension (also known as a demon dimension) was a dimension with hostile conditions for the development of human life. Demons were the dominant lifeforms in these dimensions. Earth itself was a hell dimension, as it was originally populated by demons before humans populated it and drove the Old Ones demons out.
The nature of hell dimensions also varied. Acathla's dimension was fiery and full of demons looking to torture anything with a soul. The factory dimension that Ken ruled had people working as slaves and deprived of hope of escaping.
Although most hell dimensions were described as being chaotic, some have evolved into a relatively structured society; Pylea was a feudal theocratic society, and Oden-Tal was governed with monarchy. The world of Arashmaharr was home to many sociable demons such as D'Hoffryn, who carried out their evil deeds in a business-like environment.
The demonic law firm Wolfram & Hart had its own pocket dimension, a Holding Dimension for employees who have betrayed the company; the dimension appeared as a seemingly-idyllic suburban neighborhood, where residents are regularly tortured by a demon living in their basement.
Humans who went to a hell dimension after death usually do so for specific reasons. The deceased Lilah Morgan claimed she would be sent to a hell dimension; this was due to the standard perpetuity clause in her Wolfram & Hart contract, not as punishment for her behavior in life. Rather than being subject to torture, she simply continued her employment with the company in another realm.
Similarly, when incorporeal, Spike began slipping into a dark place; this was not because of the evil he committed as a vampire, but it was due to the machinations of Matthias Pavayne, a serial killer who had spent centuries sacrificing souls to hell in order to avoid going there himself.
Alternate universes were dimensions existing in parallel with a strong resemblance to each other's realities.
The barriers between dimensions could also be breached by performing certain rituals at "hot spot" locations to transport beings and objects to and from other dimensions. For example, Pylea could be accessed in this way by opening portals with spells, while Quor'toth could only be reached by using dark magic to tear the fabric of reality.
A Pan-Dimensional Doorway ritual was powerful enough to access any dimension.