Ghost

Ghosts (幽霊) are spiritual entities believed to be the souls of the deceased who haven't passed on to the afterlife and remain in the world.

At present, the only characters known to be a ghost to have appeared in the series so far are Maya, a member of the Kamisato Faction, and possibly a girl who appears in Toaru Kagaku no Railgun: Astral Buddy.

Etymology
There are a large variety of words used to refer to ghosts in all languages, some used to refer to ghosts in general, some used to refer to specific types of ghosts, and some for both.

The term ghost itself is derived from the Old English gást, believed to come from the Proto-Germanic gaistaz. The term's current common usage as referring to a spirit of the deceased emerged during the 14th century. Aside from ghost, other terms used in English include spirit, phantom, specter/spectre, spook, wraith, haunt and apparition. A more specific term used for certain ghosts is  (German for "noisy ghost"), which is a spirit which manifests itself by invisibly moving or influencing objects.

The term yūrei (幽霊), used in Japanese, is comprised of yū (幽) and rei (霊), the latter element itself sometimes used as a term for ghosts. Other terms used in Japanese include names include bōrei (亡霊) and shiryō (死霊), as well as the more encompassing obake (お化け) which is primarily used to refer to other supernatural creatures. More specific terms used in Japanese include lit. "vengeful spirit" (怨霊), which return to avenge a wrong done to them in life, lit. "honorable spirit" (御霊), another type of typically believed to be from the aristocratic class, and jibakurei (地縛霊), which are bound to a specific location.

Principles
There are a large variety of ghost stories in the many mythologies, legends and folklore around the world.

There are similarly various reasons within these ghost stories as to why a ghost remains in the world and doesn't pass on to the afterlife. In some cases, there is some lingering regret or grudge which ties the ghost to the world, the nature of their death prevents them from passing over, and for some, such as, they made transgressions in life which have left them unable to enter the realms of the afterlife and forced them to wander the Earth.

Various cultures have common appearances associated with ghosts. In Japanese culture for example, ghosts are depicted as wearing pure white burial clothes with a triangular headdress, often hunched over, posessing long, black, disheveled hair, and accompanied by.

Common attributes often associated with ghosts include, varying degrees of transparency and pallid complexions. In various stories, they are often accompanied by cold temperatures, pale floating lights or embers (referred to as, hitodama or ghost lights), and mist. Some ghosts are said to possess the ability to move or influence objects to a certain extent, and a few are thought to have the ability to objects or people.

Certain locations are said to be haunted by ghosts, including certain graveyards and. These locations might be inhabited by spirits for a number of reasons; the ghosts may have been former residents or familiar with the location in life, it might be the resting place of their remains, or it might be the site of their death (in a fair number of ghost stories, these deaths are violent or tragic).

There are various stories of ghosts who haunt or curse people who enter their domain or commit a transgression against them. Ghosts are sometimes given prayers and food offerings, or have ceremonies conducted in their name, to honor or placate them. Some ghosts have shrines dedicated to them, and a number of them are even worshiped as deities.

There are a number of Urban Legends associated with ghosts, including, and ghosts who don't realize they're dead and continue trolling message boards.

Together with the ghost stories found across many cultures, there are also a fair number of people who possess a.

Magic
The existences and principles behind ghosts held on the magic side have not been fully explored, however there are various forms of magic which make use of legends and folklore surrounding ghosts.

The Russian Orthodox Church's Annihilatus specialize in disposing of spirits such as ghosts, including will-o’-the-wisps, specters, pregnant spirits and other sorts of things that shouldn't exist within the perimeters of Christian lore. To Annihilatus, a wandering spirit should be eliminated, as it is "something that should not exist" in this world, as the soul is meant to go to heaven, hell or the purgatory, and not remain in the world. Annihilatus normally forcibly erases demons, ghosts and other such phenomena rather than trying to "send them off" to the next world in a peaceful manner, regardless of their circumstances. Members of Annihilatus carry weapons and interrogation tools that are powerful against malign spirits, though they are not particularly suited for interrogating and hunting down humans, if ever the situation calls for it.

When carrying out certain forms of Necromancy to put back the wandering "residual memory" of a corpse and close their "spiritual circuit" in order to put the deceased to rest, if their "residual thoughts" (残留思念) are not controlled, they can take a dangerous form which is sometimes called a ghost (地縛霊), gathering outside the corpse and appearing in the form of a floating brain. If someone touches it, the memories of the deceased will flow into their mind.

Science
The existence of ghosts is generally not accepted within scientific circles, however there are a number of theories whispered about for a scientific explanation for ghosts. These include plasma, cognitive disorders and magnetic abnormalities.

One of these theories is that of extremely low frequency waves outside of the audible range; when people are exposed to stimuli they can't process with their five senses for long periods of time, they become unable to relax and end up seeing or hearing imaginary sights and sounds. Ghosts are often seen in abandoned houses and caves because they tend to have drafts and creaking floors. The accuracy of this theory isn't fully known, however Maya makes use of it in her Censers in order to help sustain her existence.

RSPK Syndrome is also referred to by the term Poltergeist (乱雑解放 (ポルターガイスト) ) as the manifestation of the phenomena is similar to the actions of a poltergeist of lore, like moving objects and other unnatural occurrences.

Examples
Maya, the first ghost to appear in the series, doesn't know what a ghost or a soul really is, but pictures it as something like a smell; if she can rub the smell of her life onto something before it fades with time, she can live on and if she can't produce the original smell, she has to protect the current one, as once it disappears it would be the end for her.

Lacking a physical body, Maya makes use of Censers, drones which emit imperceptible low frequency waves and aromas, to create an artificial haunted location to sustain her existence and exert corporeal influence. She also maintains her own scent through eating vegetable sticks and sipping sweet drinks. As well as being supported by a scent, her body also produces a scent, creating a cycle which can be built up to run in a manner similar to that of a. By altering the mix and distribution of these parts, the scent and the 'haunted location', compared to a prism that separates light and how the status of a ghost in a ghost story depends on how the story got started and where it came from, Maya's form can be changed, from something like a harmless wandering ghost to a gigantic destructive spirit.

In the case of the girl encountered by Hokaze Junko, she doesn't have any memory about herself but given her ghostly characteristics and circumstances, considers it likely that she is the ghost of someone who has died. She has displayed the ability to possess living beings, a trait associated with certain ghosts, as well as incorporeality, floating in midair and being visible only to Junko.

Shinyaku Toaru Majutsu no Index
The first known ghost to appear in the series is Maya, a member of the Kamisato Faction, who bears the traditional appearance associated with Japanese ghosts.

Toaru Kagaku no Railgun: Astral Buddy
Around the same time as the events surrounding the Dream Ranker, Hokaze Junko encounters a girl who displays the characteristics of a ghost.

Trivia

 * As well as specifically referring to the spirits of the dead, the term 'ghost' has sometimes been used to refer to figures from the past that continue to linger or seem to appear when they should be dead, such as Aleister Crowley (who had faked his death and is believed to be dead to most of the world).