Dimension

A dimension (also called plane, world, or realm) is a self-contained universe. Together all dimensions constitute the multiverse (or Buffyverse).

Concept
Earth can refer to the planet Earth, but also the universe around it, including other plans, the Sun, the galaxy, and faraway stars. There are thousands, perhaps millions, of other universes like this. All these universes are unique in their own way. While some dimensions are very similar to earth, like the Wishverse, others are widely different like Quor'toth or World With Nothing But Shrimp. Each dimension can have different laws of physics and different laws of magic. The size of dimensions also varies, while some dimensions contain planets, stars, and galaxies, others contain only a single room, or a black void of unknown size.

Time in different dimensions
Each dimension has three spatial axis, allowing creatures to move up back and forth, left and right, and up and down (as far as gravity allows). Each dimension also has one temporal axis (time); while most creatures (like humans and vampires) travel along this axis at a constant speed (it takes them one second to move one second in time and one day to move one day in time), some powerful creatures (like Illyria) can freely move back and forth at any speed. The flow of time within hell dimensions is often different than that on Earth; for example, Angel was trapped in Acathla's realm for at least three months in Earth time, but was said to have endured hundreds of years of torture, and Connor returned from Quor'toth in his mid-to-late teens after having entered the dimension as an infant mere weeks beforehand. 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.

Types of dimensions
The most prominent distinction between types of dimensions is the distinction between heaven and hell dimensions.

Hell dimensions
A hell dimension (also known as a demon dimension) is a dimension with conditions extremely hostile for the development of human life. They are known as "hell" dimensions due to their similarity to the nightmarish afterlife believed to exist by many religions. Demons are the dominant life form in these dimensions and many of these worlds are inhabited by pureblood demons. Earth itself is technically also a demon dimension, as it was originally populated by demons.

The nature of hell dimensions also varies. Some, such as Acathla's realm, are implied to resemble the traditional view of Hell; fiery and full of demons looking to torture anything with a soul. Others are more down to Earth, but just as twisted; such as a dimension where people are worked as slaves and deprived any hope of escape. The demon Sweet came from a presumably musical hell dimension. Although most hell dimensions are described as being chaotic, some have evolved into a relatively structured society. Pylea resembles a medieval Earth full of demon champions and princesses. The world of Arashmahaar is home to many sociable demons such as D'Hoffryn who carry out their evil deeds in a business-like environment. The demonic law firm Wolfram & Hart has its own pocket dimension to hold employees who have betrayed the company; the dimension appears as a seemingly-idyllic suburban neighborhood, where residents are regularly tortured by a demon living in their basement.

Heaven dimensions
A heaven dimensions (also known as q higher plane) is a dimension which is populated by benevolent beings. They are known as "heaven" dimensions due to their similarity to the idyllic afterlife believed to exist by many religions. Little else is known about heaven dimensions.

Afterlife
Despite the belief of many religions that virtuous people will go to Heaven and sinners will go to Hell, there is little concrete evidence of this.

Humans who end up in 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 because of any sins she committed in life. Rather than being subject to torture or punishment in hell, it seems she is simply continuing her employment with the company in another realm. Similarly, when incorporeal Spike began slipping into a dark and hellish dimension, 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.

After her death, Buffy Summers, a heroic champion, did go to a heaven dimension, which she confirmed was populated by the souls of deceased humans. It was unknown whether these were only souls of people who had been good or not. What actually happens to the souls of people who have committed evil is never confirmed, although the poltergeist James Stanley was seen entering a heavenly afterlife after finally being forgiven for killing his lover, suggesting that they are not necessarily damned.

Alternate universe
The term alternate universe is very ambiguous; it is often used to describe worlds that are very similar to Earth, and thus also inhabited by the same people, but with minor differences (like as Wishverse or the world where Buffy Summers was in an asylum). Alternate universes can be interpreted in two ways; it is not clear which is true:
 * Some suggest that whereas true dimensions are entirely different worlds, alternate universes are merely different forms of the same dimension. As such each dimension most likely has a number of alternate forms (for instance, Quor'toth might have an alternate form where Connor never came).
 * Others interpret alternate universes as regular dimensions which merely have a strong resemblance to each other. This means which dimensions are considered alternate universes of each other is merely a subjective perception of individuals.

Interdimensional travel
There are various ways of travelling between dimensions:
 * Areas in which the barrier between dimensions is thin is known as a Hellmouths; a mystical convergences which emit supernatural energy and attract evil.
 * Similarly, the barrier can also be breached by performing certain rituals at "hot spot" locations.
 * An ancient, mystical force known as The Key is capable of breaking down these barriers completely, causing all dimensions to bleed into one another and chaos to reign.
 * The original full version of Spike's ship was capable of interdimensional travel.