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, two characters known to be ghosts to have appeared in the series: Goryou Maya, a member of the Kamisato Faction, and Frillsand, a scientifically created artificial ghost.

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, possessing 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 as the manifestation of the phenomena is similar to the actions of a poltergeist of lore, like moving objects and other unnatural occurrences.

Drencher Kihara Repatri developed artificial ghost technology, used to produce Frillsand and reverse-engineered by Kihara Hasuu to persist after his own death, which is based on a method known as High Voltage Cutting (ハイボルテージ＝カッティング法). The method uses principles similar to those responsible, ion sheathes and , with a constant emission of powerful energy eventually creating irregular waveforms and images while interfering or overlapping with itself,  and that phenomena being intentionally created and manipulated in order to make something nonexistent appear in the physical world.

In describing the principles behind High Voltage Cutting to Hamazura Shiage and Takitsubo Rikou, Frillsand #G noted that the most common types of haunted locations (e.g. old homes full of drafts, cliffs and caves eroded by waves, mountain roads late at night etc) were often full of noise (e.g. static electricity, pressure differences, creaking rusty doors, the rustling of trees in the wind etc). She also asked in passing if they had even heard a ghostly voice in the crackle of excess electricity or in the roar of hydrogen gas being ignited. Drencher had previously visited many so-called haunted locations worldwide and revealed the scientific causes of strange phenomena there, artificially recreating these things later to use for his own purposes.

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.

The artificial ghost Frillsand (as well as the ghost of Kihara Hasuu, who reverse-engineered her structure to persist after death) is an existence of unstable electrical energy formed using the method known as High Voltage Cutting, based on manipulating a constant emission of high energy and controlling the irregular forms produced from that power interfering with itself. Frillsand #G's power source is a 'civilization battery', energy present on the scale of a metropolis, constructed from a combination of copper, zinc and other metals present where people live,  and acid rain produced by carbon dioxide and nitrogen oxide constantly produced by human society. By absorbing power from a 'battery' the size of a city or greater, a single outlier point was constructed within the stable energy. Frillsand #G has been said to be capable of existing so long as human civilization exists and not cease unless it is eliminated.

Artificial ghosts of this type are described as physical phenomenon electrically explainable through High Voltage Cutting and supported by chemistry and physics, and are apparently not suspectible to Imagine Breaker. Being collections of electrical energy, they are capable of shocking and burning in a similar manner, and being supported by electricity, they have corresponding weaknesses, such as being affected by electromagnetic pulses and similar electromagnetic wave emissions. As masses of unstable strong power, they are the opposite of aggregations of weak powers such as AIM diffusion fields, resulting in incompatibility and malfunctions when the two come together.

Frillsand #G was designed to possess several traits which have been associated with ghosts. As an incorporeal ghost, she can pass through physical objects and vice versa, but she also cannot touch them. Reproducing the harm that is said to befall those who encounter ghosts, she can remotely and continually damage living people who encounter her if they see her. In a similar manner, she can induce malfunctions in mechanical lenses and sensors, preventing them from properly recognizing her. While he copied her basic structure, Hasuu discarded these features in order to mitigate compatibility problems with the Vanishing Tunnel.

Other
There are a number of entities, phenomena and subjects on both magic and science sides which are similar to ghosts in certain ways but are also different in certain ways.

In the case of the girl encountered by Hokaze Junko, she didn't have any memory about herself but given her ghostly characteristics and circumstances, considered it likely that she is the ghost of someone who has died. She 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. She was even referred to as the 'ghost girl', while her real name was still unknown. Eventually, she was revealed to be Yuuri Senya, an esper with the ability Astral Buddy, whose projected consciousness had been separated from her comatose body since a disastrous experiment at Clone Dolly Ideal.

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.

Operation Handcuffs Arc
Frillsand, an artificial ghost created by Drencher Kihara Repatri through application of High Voltage Cutting, played a considerable role in the events surrounding Operation Handcuffs.

Post-Handcuffs Arc
The events which transpired on December 29th in the wake of Handcuffs, including the Overhunting crash and prisoner escape and the battle to rescue Risako in the Vanishing Tunnel, involved artificial ghosts produced by High Voltage Cutting, both Frillsand #G and Kihara Hasuu, who had reverse-engineered the technology behind her and managed to use it to persist as a ghost after his death on December 25th.

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).