Cloverfield

Cloverfield (長井空間(クローバーフィールド)) is a term used by the Toaru Majutsu no Index franchise fandom to refer to Nagai Tatsuyuki's style of direction in the original anime episodes of Toaru Kagaku no Railgun S. It is most often used as a derogatory term by the fandom due to the perceived low quality of the episodes, though it is also used to refer to complaints of the Toaru Kagaku no Railgun anime in general.

Terminology
The term cloverfield is most likely derived of the abundance of clovers in the Toaru Kagaku no Railgun S as a theme. The term "Nagai Space" comes from Nagai Tasuyuki's given name. The first appearance of clovers occur in episode 7 of the anime right in the middle of the Sisters Arc, where most of the plot focuses on not Misaka Mikoto but on Shirai Kuroko and her attempts to find a four leaf clover as a good luck charm in Academy City.

Principles
Tatsuyuki Nagai is an anime director mostly known for his work on drama such as Toradora and Ano Hi Mita Hana no Namae o Boku-tachi wa Mada Shiranai, and has notably directed Honey and Clover II, a series with clovers as a main theme. His style shows prominently in the Railgun anime. In Toaru Kagaku no Railgun though some dramatic scenes are present in the original manga, the anime adaptation has them moved around, allowing for an episode focusing entirely on character drama instead of the story, making for the anime having a much more subdued pace. An example of this is episode 2 of Toaru Kagaku no Railgun, whose content was taken from chapter 17 of the original manga.

However, most of the focus of the term is derived from the anime's original episodes, which many of the fandom believe to be of low quality due to their story, pacing, characterization, or contradictions of established canon. Prime examples of these contradictions involve Mikoto asking aid from her friends, one of whom is a powerless Level 0 civilian, to aid against STUDY, a group that is part of the dark side of Academy City, as well as the final episodes of the Railgun anime, where the friends gather to resolve the storyline and defeat the antagonist. This despite the fact that a previous Toaru Majutsu no Index storyline, which chronologically proceeds both of the above examples, has Mikoto going alone and being unwilling to involve Shirai Kuroko. In fact, the villain of that arc, Musujime Awaki, mocks Mikoto for involving Kuroko, despite Kuroko being the one who involved herself in Mikoto's affairs.

Another complaint is the inclusion of powerless characters, specifically Saten Ruiko, in dangerous situations, despite her lack of actual combat ability and civilian status. This extends to her being part of the resolution of a conflict, which is perceived by the fandom as contrived and to the detriment of established rules or logic: that instead of her working around the rules established by the setting, it is the setting that bends for her. For example, in the first episode of Toaru Kagaku no Railgun S, a ladder connecting the top floor of the hospital to the roof conveniently appears nearby a window through which Ruiko is peering. This allows Ruiko, in an event of pure serendipity, to save Haruue Erii from falling off the roof after the thugs that kidnapped her awkwardly threw her an improbable distance from a helicopter. An extension of this is other characters being shoehorned into the story, which the fans perceive to be contrived or forced for the sake of what they call the "power of friendship".

Other reasons are as follows:
 * The creation of the four girl ensemble between Mikoto, Kuroko, Kazari, and Ruiko that was not present in the original manga.
 * The cluttering of the Toaru Majutsu no Index timeline through original episodes with multiple days passing each episode. This extends to the Toaru Kagaku no Railgun (PSP game).
 * The butchering and flanderization of character personalities and like Shirai Kuroko, Misaka Mikoto, and Kongou Mitsuko. An example of this is Uiharu Kazari blatantly ignoring her duties as a Judgment in order to protect Haruue Erii.
 * The usage and rearrangement of material from the manga that does not fit with how it was originally presented in the manga. An example of this is the early introduction of Shokuhou Misaki in episode 1 Railgun S. Another would be the adapting the clearly gag-centered 4-koma omake from the manga of Kuroko and Frenda's meeting into the anime.
 * The repetitive use of gags and establishing scenes. An example of this is Kuroko molesting Mikoto and characters eating things.
 * Excessive focus on characters irrelevant to the plot, such as Haruue Erii and Edasaki Banri.
 * Excessive melodrama, such as Erii and Kazari crying over the former moving despite being able to easily see each other again as well as the fact that the two of them lived together for a short period of time.
 * The weakening and giving characters "plot-induced stupidity" in order for other characters to come another's aid. The most blatant example is Mikoto allowing herself to be injected with an unknown, incapacitating drug as a precondition for a powerless man, Aritomi Haruki, giving her an important plot device after being warned that her powers might cause harm to it. This in spite of Mikoto being physically capable of taking people on her own, not needing to directly apply her powers on Haruki, having a physically capable Nunotaba Shinobu with her, as well as Haruki being mere feet from them.
 * Anime-original content for the sake of providing happy endings. See the anime adaptation of the Level-Upper Arc, where Kiyama Harumi and her students are saved.

Spread
The term, in Japanese, appeared on Japanese blogs as early as September 7 of 2013, an entire day after the airing of episode 21 of Toaru Kagaku no Railgun S, the very episode where Mikoto asks for her friends for help. The term later jumped into the Western fandom on September 10 after it was translated on 4chan's anime board. On September 14, the Index atwiki published an article regarding the term.

Trivia

 * Perhaps the term being a further reference to it, Nagai also directed ,