Magic

Magic (魔術) is a term used to refer to supernatural phenomena that cannot be explained or analyzed via any sort of scientific means and reasonings. In the Toaru Majutsu no Index universe, magic is a complex system of arts, disciplines, and traditions that are firmly grounded in laws and knowledge that are wholly unknown and beyond the grasp of human sciences, which sought to manipulate and distort certain aspects of reality.

Also called the occult, conflicts with the magic side, the secret underworld that deals with magic and the occult, are the primary and most recurring plot points of the series.

History
Magic by itself roots from the simple fact that 'those without talent' wanted to be able to do the same things as 'those with talent' - namely Esper and Gemstones.

Principles
In contrast to espers who have only one ability that is decided by birth, magicians can create many supernatural phenomenon without restriction by birth. But they have to take time and effort.

Mana
Mana is the energy that allows magicians to use magic. Mana exists in all forms of life, not just magicians, although they are the only ones who use it. It has been explicitly stated one method of creating mana is by processing a person's inner lifeforce. The various schools of magic, for example Nordic and Shinto styles, requires different 'kinds' of mana for casting that other styles might not use.

There is 'natural' mana that exists within the earth itself, which some magicians or magical objects, such as the grimoires, draw their power from, in the form of "leylines". Mana is incompatible with the AIM fields that espers produce, as a result, whenever an esper performs magic he/she becomes severly injured.

Mana can be "inserted" into objects following complicated rituals, which can give birth to "magical items" that posses magical properties on their own, for example in one opportunity Index references the ritual needed to add power to a holy cross.

Experts theorize that since Mana comes from the caster's life force, then immortal beings would possess an infinite amount of Mana, having access to unlimited and powerful magic that no magician would be able to fight against. This is one reason why Vampires are thought to be invincible by various people on the Magic Side.

Ley lines
A ley line is a current of power that runs through the earth, using a number alignments using terrain and monuments, among other geographical interests. Ley lines are alterable by destroying and altering the terrain that runs through it, or applying the art of feng shui. However, modification of ley lines is a delicate process, as making a mistake can cause disastrous results. As such, modifications of ley lines are often carried out as big national projects and magicians who use earth for their magic are forced to be careful lest they destroy important magical points.

With modifications, leylines can be used to strengthening the power that flows through a ley line, or as something akin to pipelines. Ley lines can be described as pipelines and the nodes where they meet up for example, the Avignon and the Vatican, act as reservoirs where other pipes are connected. With this, certain affects can be achieved, such as a spell travelling through the ley line from a node to another node, which will then spread that spell's effect through the ley lines it is connected to.

Styles
Many of the magic spells etc. is based on a certain set of laws - e.g. Celtic or Norse. But there are no strict division, because one base can be based on another base - e.g. Norse mythology was influenced by Celtic culture. However, magic users often use magic that is inherently part of their culture, like Kaori using her Amakusa spells based from Shinto, Buddhist, and Christian origins.

In the end, religion plays a key role in magic, as cultures and traditions of a certain population are tied to it. For example, Agnese Sanctis once belived that all magic that does not stem from Roman Catholicism is black magic.

Western magic
As the West is dominated by Christianity, most magic users are influenced by it one way or another, alongside whatever cultural traditions they've grown up with. This also includes pagan or pre-Christian origins, for example the Roman Catholics, despite their stringent adherence to the use of their own magical traditions, used Wooden Stake magic, the origins of which came from the Pythagoran Order. As well as the magic names used by English magicians used Latin

Organizations
Magic is used to realize a certain goal. So all magicians wield their power for themselves to realize this goal. These goals are carved into themselves in Latin - Magic Names. Magicians often use an organization like magic cabals or denominations to realize their goal.