Magic is a supernatural force used by most of the Exceed. The Exceed are ranked based on their innate talent for it.
Spirit Circuits are naturally present in all Exceed besides Imanity. They allow their possesor to touch the origin of magic in order to use it, and to sense its use. It is unknown whether they are physical pathways throughout the body, or metaphysical pathways of energy, like chakra points in Hindu lore. However, the highest level Exceed don't actually posses physical form, so it can be assumed that they are non-physical. It is unknown if they can be bet in a game.
The use of magic is undetectable to those without the ability to use magic themselves, however, its use causes the eyes of the user to glow in a fashion similar to Sora's eyes when he grins slyly.