Mana

Mana is the energy that flows across all the planes of the universe, and the power that is harnessed by spellcasters in order to use magic and cast spells. Mana comes from the Arcanum Circular, which flows from the Essential Plane all the way up to the Infinite Skyscape, and all the way back down again.
Mana is an integral part of existence. All matter is naturally imbued with mana, which allows them to hold shape, and allows for spells to interact with the world. However, too much or too little mana will cause matter to grow unstable: in the metaphysical plane, where there is an excess of mana, matter is transient, ever-changing, and volatile. In the essential plane, where mana is actively absorbed back into the Arcanum Circular, matter is unable to hold its form, and will be ripped apart. The material plane lies in the middle, allowing for matter to hold its shape consistently, but limiting the amount of spellwork that can be done upon the world.
Beings with awakened cores will naturally draw in mana from the environment through a process called absorption, up to their mana capacity. As mana passes into one's core, it will automatically take on one's natural aspect, though one can channel it into a different aspect if needed -- as long as it is not their natural counter-aspect.
When mana is released from the body into the world, it will take some action upon the world, according to its shape. A spell is a particular shape of a particular aspect of mana; mana improperly shaped, or not shaped at all, is considered 'wild,' and will have untold effects once released.
Once mana has run its course on the world, it will leave behind magical residue. This residue will retain the aspect it once had, though not the shape, and is detectable. Over time, the residue will be absorbed back into the Arcanum Circular and take to the essential plane, where it will be broken down and recycled into mana once again.
Due to the cyclicar nature of the Arcanum Circular, mana cannot be created nor destroyed, and there will always be a finite amount in the universe, either existing as free mana, stored mana, or magical residue.

Technicals

The standard unit for measuring quantities of mana is a 'mana point,' or 'mp,' and the measure of mana points/second is known as a 'thaum,' or 'xm.' A being's mana capacity can be measured in mp, and their average absorption rate in xm. The amount of mana a spell takes can also, hypothetically, be measured in mp and xm, but those numbers fluctaute: for example, the spell Levitate uses mana at a different rate depending on the weight of the object being levitated, the speed at which it is being moved, and the distance from the caster. So, while you could say it has a rate of .4xm for a 1kg object being moved at 1m/s in a circle of radius 3m about the caster, and that would always hold true, there are far too many variable for that to be a reliable method of calculating how costly a spell is.

Other magical concepts:

- Aspects

- Cores