Magic Circle

A Magic Circle (魔方陣) is a design or arrangement constructed to facilitate the use of Magic.

Principles
There are various elements which can be used within a magic circle and various forms which they can take - the simplest form is that of a lone circle.

Magic circles often include magical symbols in order to draw power from what they represent via Idol Theory.

Magic circles can be created through various means, from drawing designs and arranging certain objects, to using the positions of the stars themselves.

Orsola Aquinas Rescue Arc Arc
During the battle with the Agnese Forces at the Church of Orsola, the Amakusas placed Stiyl Magnus's rune cards in specific positions across the church's grounds in order to form a large magic circle and enhance the power of Innocentius.

Daihasei Festival Arc
During the events of the Daihaseisai and their pursuit of Oriana Thomson, Stiyl Magnus and Tsuchimikado Motoharu explained the basic principles behind magic circles and grimoires to Kamijou Touma.

Trivia

 * One particular magic circle is featured prominently in the anime and manga adaptations of Toaru Majutsu no Index, as well as their promotional materials and related media. It consists of a circle containing a pentagram, with a smaller circle circumscribed by the center of the pentagram. Several sets of letters are arranged around the perimeter of the circle, with crosses separating them. In some versions, two larger concentric circles surrounding the base circle and letters. The sequence of letters is as follows (clockwise); "ISCATC+HFIMMMP+MNIP+MNIA+ICY+ISWS".