Mana

Mana (魔力) is the energy that allows magicians to use magic.

Principles
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 (Life Force (生命力)), similar to how crude oil is refined into other substances. 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 possess magical properties on their own, for example in one opportunity Index references the ritual needed to add power to a holy cross.

It has been stated that as a consequence of having trained their magical powers, Magician's Mana is so much stronger than the average person that a Magician can easily tell who is a Magic user and who is not just by standing close to them.

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.