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 his style shows 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 episode 2 of the 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 believes to be of low quality due to in part because of its story, pacing, characterization, or contradictions to established canon. A prime example of this, most often used by the fandom, is Mikoto asking aid from her friends, one of which is a powereless Level 0 civilian, to aid against STUDY, a group who is part of the dark side of Academy City. This despite the fact that a previous Toaru Majutsu no Index storyline, which chronologically proceeds from the above one regarding STUDY, has Mikoto going alone and 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, in spite of her lack of actual combat ability and being a civilian as well. This extends to her being part of the resolution of a conflict, perceived by the fandom as contrived to the detriment of established rules or logic, that instead of her working around the rules set up by the setting, it is the setting that bends for her. For example, the sudden appearance of a maintenance ladder nearby a window where Ruiko is conveniently peering, from the very the very first episode of Toaru Kagaku no Railgun S. This allows Ruiko, in an event of pure serendipity, to save Haruue Erii from falling off the roof after the thugs that kidnapped her somehow was able to throw in awkward position. An extension of this are other characters being shoehorned into the story, which the fans perceive to be contrived or force for the sake of what they call "power of friendship", and points out to the finale episodes of the Railgun anime, where the friends gather to resolve the storyline and defeat the antagonist.

Other reasons are as follows:
 * The creation of the four girl ensemble 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 not part of 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 a drug that doesn't know of its contents to allow a powerless man, Aritomi Haruki, to give to 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 also a physically capable Nunotaba Shinobu with her, as well as Haruki being only a feet away from them.

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.