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.

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.

The Guild
"The Guild" (ギルド) (actual name currently unknown) is a guild of magicians based in Europe.

The Guild had a base on the Greek island of, had members throughout Europe, and was known enough in that "the Guild" would be sufficient for people there to know which group was meant. Given its history, the group and many of its members used Greek, Roman and Christian elements in their magic and methods.

The Guild's likely leader is an individual referred to by codename in the group's records (and the only one to be referred to in this way) as Tu.

Toaru Majutsu no Index SS: Agnese's Magic Side Work Experience
"The Guild" were responsible for enabling a number of magical incidents across Europe which Agnese Sanctis, Lucia and Angelene were involved in resolving. 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, though the group's main force was still at large.

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.