Guild

A Guild (ギルド) is an organization which is a collection of individuals specializing in a particular field, gathering to advance their skills and the field in general.

Principles
Similar to lodges and salons, the term guild typically refers to a collection of craftsmen or intellectuals with a certain specialty.

On the underside of society, guilds can be gathering points for workers who cannot openly advertise their services, such as an illicit manufacturing guild (skills and services might include procuring materials, manufacturing, shipping products, sales etc) or an assassination guild (skills and services might be include procuring weapons, carrying out a kill, disposing of bodies, making alibis etc). In this case, to avoid detection, guild members are supposed to remain as inconspicuous as possible, using greetings, handshakes, cards or meeting places which only other members would recognize, with the guild's base being the biggest secret. Being nondescript is considered the most notable signature of this type of guild.

Guild is described as being a fairly common term for those steeped in the magic side, a magician's guild being similar to normal ones but focused on magic and with distinct differences from a magic cabal. At present, only one magician's guild, actual name unknown and referred to just as "the Guild", has been shown.

Background
One notable magician's guild, dating from the time of ancient Greece, managed to survive to the present day by adapting to match the changing times in Europe, working within a Greek then a Roman then a Christian society. From the absorption of the Greek lands and culture into Rome, overwriting Rome's indigenous mythology in process, and the oppression of early Christianity by the Roman Empire followed by the oppression of all other religions once it was accepted, the Guild blended in with the masses, freely changing out their religion and culture to match the side which was currently winning.

Toaru Majutsu no Index SS: Agnese's Magic Side Work Experience
A large magician's guild dating from the time of ancient Greece, referred to only as "the Guild", were responsible for enabling a number of magical incidents across Europe which Agnese Sanctis, Lucia and Angelene were involved in resolving,   as part of their plan to bring about the destruction of Christian society and move on as they had done before. After discovering their existence through investigations in Zurich and Turin (in part due to encounter with a former acquaintance, Monica, who had since been recruited by the Guild and was spying on them), the nuns and other Anglicans attacked and seized the Guild's base on the Greek island of, obtaining records needed to hunt down its members across Europe and around the world, though the group's leader and main force was still at large, and proceeding with a more direct approach.

After some investigation into a destroyed statue on Thira, a series of museum thefts and the demise of a bloodline investigator, the nuns and the Anglicans were able to learn of Guild leader Tu's identity as Albert Dying and the Sickle of Saturn, but the Guild had already completed their preparations and shortly afterwards, Albert used the sickle in the heart of the Vatican. Although the sickle's influence immediately engulfed the Italian Peninsula and quickly began to spread through Europe, the Vatican's defenses blocked it from taking over the Vatican itself, leading Albert to attempt to kill the Pope to remove the core of these defenses. However before he could do so, he was confronted and defeated by Agnese, with help from Lucia and Angelene, who subsequently destroyed the sickle.

Coronzon Arc
While observing the final barrier of the England-Londinium Fortress, described as giving physical repulsive force to rules permeating the land and space, and using aspects of national or regional character which work to distance outsiders, Index reckoned that it probably originated with guild membership rules which forced members to protect the group's secrets.