Sigil

A Sigil (シジル) is a magic symbol used to represent certain subjects according to Idol Theory. Most often in Western magic, it represents an angel.

Principles
Sigils represent things from a special cross, to that of Hebrew alphabets. The lines used in a sigil are used to display a word. However, it is most often used to display a name of an angel in order to obtain Telesma. What the sigil draws out depends on the angel the sigil represents, like Archangel Gabriel representing the direction of south, which the spellcaster may use to tell others which direction to go.

What they represent seem to imply that they can also be used in magic circles, for when writing the name of the angel whom the spellcaster desires to borrow the from, among other functions, instead of regular alphabets.

As Silvia demonstrates, there is no reason for a sigil to be solely written on paper or on the ground - they can also be formed through various movements, like using a rope to signify the sigil of Archangel Gabriel.

Sigils are incorporated into the internal 'contents' of the Anti-Art Attachment's components, and sigilized names might be carved on surface of Ange Catacombs' autonomous suitcases.

Toaru Majutsu no Index: Mars SS
After being taken hostage by Bisai Otome, Index left behind sigils to tell Kamijou Touma and Fleiss which direction to go in order to follow her.

Toaru Majutsu no Index: Kanzaki SS
During the confrontation between Kanzaki Kaori and Leivinia Birdway as the two were going after Orlentz Trice and losing their tempers, Birdway ordered Mark Space to bring her a magic sword, intending to take Hebrew text from a cross medal and draw a sigil to use against Kanzaki.

Ichihanaran Festival Arc
Silvia made use of sigils during her battle with Thor.