Alien

Aliens (宇宙人), also referred to as extraterrestrial life-forms (地球外生命体), are theorized life-forms which do not originate from planet Earth. Although humanity has yet to encounter extra-terrestrial life, aliens feature prominently in science fiction, conspiracy stories and other parts of popular culture.

Although no truly extraterrestrial life-forms have been canonically confirmed within the Toaru series, aliens and various things associated with them have been referenced numerous times within the story.

Etymology
The term Alien is derived, through Middle English and Old French, from the Latin aliēnus ("belonging to another, exotic, foreign") itself derived from alius ("other").

Aside from being used to refer to extraterrestrials, the term can also be used to refer to an outsider and foreigner residing in a country, a life form from outside a considered family, group or territory, someone excluded from certain privileges, or a quality of something very strange and unfamiliar.

Principles
Among the many theories which have arisen surrounding the identities of Fräulein Kreutune and Aiwass are theories that they are aliens. A number of the supposed phenomena which have been attributed to aliens, such as abductions and, have in the past been associated with supernatural entities such as fairies.

Aliens often appear in modern science fiction and popular culture, and there are various fictional characters who are aliens. One such character is the British mascot Bunny Grey.

Toaru Majutsu no Index: Mars SS
The conflict between Bisai Otome and Fleiss, which Kamijou Touma and Index got caught up in, involved the Attached Microbes, supposedly an intelligent life-form native to Mars, and the Mars World experiment to recreate the microbes and their environment.

Baggage City Arc
One of Kihara Byouri's Dark Matter-based transformations, Reference: Little Grey, was based on a alien.

Processor Suit Arc
During its battle with Coronzon, Aiwass made use of the theory of it being an extraterrestrial life-form to compensate for the lack of an avatar as they entered outer space aboard the Windowless Building.

Coronzon Arc
One of the Crowley's Hazards involved in the invasion of London had the appearance of an alien with an oversized head. During the same incident, Lessar and a female knight were scared by Karasuma Fran descending from her illuminated UFO balloon like an alien. The associations between UFOs and crop circles and theories of aliens building ancient civilizations were referenced at several points by several individuals during the incident.

The next morning, when Coronzon was attacking locations remotely through the ley lines to stoke the chaos across the world as part of her preparations for the Ceremony of Mo Athair, at least one witness referred to the belief that aliens constructed ancient monuments.

Post-Handcuffs Arc
On December 29th, while guiding Anna Sprengel around her group's base and warning her not to upset Alice, Aradia referred to how the public was talking about the discovery of water and traces of life on a planet dozens of light-years away, which had got there due to an incident where Alice got upset and started throwing her things around, describing reality as a fickle thing.

Toaru Kagaku no Railgun SS: Liberal Arts City
The Laveze Squadron involved in the Liberal Arts City incident were named after aircraft from the movie "Alien Wars" modified to fight UFOs.

Motto Marutto Railgun
Aliens, particularly the greys, were brought up numerous times in sketches where Misaka Mikoto, Shirai Kuroko, Uiharu Kazari and Saten Ruiko discussed conspiracy theories and the like. Uiharu also thought of an alien kouhai when she expects new members of Judgment at their branch.

Trivia

 * According to Haimura Kiyotaka, he designed Aleister Crowley to be something "like an alien in human skin."
 * One of the unused designs by Kiya Shii for the dragons which emerged after Kamijou Touma's right arm was severed in an incident during the Daihaseisai is for an "extraterrestrial" one.
 * The premise of an interview with Kamijou Touma and Misaka Mikoto in Dengeki Splash!, gradually revealed throughout the course of the interview, is that Mikoto had apparently caught and defeated an alien trying to swap places with Touma, and had come to the beach with him(?) in order to bury the body. The ending of the interview implies that the aliens were not as defeated as she thought they were.
 * While serializing the climax of Jailbreaker Arc, Ogino mentioned the four girls meeting a self-proclaimed alien living in a steel tower for a possible future plotline.