Potentia, sometimes colloquially called mana, is a form of potential energy which can be used to fuel magic. Unlike other forms of potential energy, such as chemical, elastic, or gravitational, Potentia is sensitive to the will of conscious beings like dragons, sapes, and nature spirits. Potentia is measured in joules.
Magic users can store magic, though the physical location which stores this magic does not appear to be located inside of the physical body, or within three-dimensional space at all. A hypothetical organ called the olum, which is integrated with the digestive system of each magic user, is believed to be the storage location for potentia.
In normal circumstances, potentia is replenished at some rate by the surrounding leylines, at a rate determined by proximity to the leyline, proximity of that leyline to its source, potency of that source, and draw rate of the magic user.
In extreme cases, the user may draw potentia beyond what is available to them through will magic. But this is a gamble which may lead to olum inversion, a potentially deadly condition.
Potentia is used in accordance with the intensity of the spell being cast, with an efficiency rate determined by the degree of knowledge the user has on the domain of the spell.