Mana

Mana is everywhere, in everything. This world is infused with it, from the stars above, to the depths of the Great Ocean. Some believe the world was birthed from it, and I wouldn't be surprised if that particular legend were true.

- (A smart mage character)

Mana is the term used for the mystic energy that suffuses the world. It is ever-present, and able to provide fuel for impossible actions, such as rapidly healing wounds, or throwing a ball of light that explodes into a blast of flame, to those willing to learn how to channel it.

Origins

According to ordinary people living in the Material Plane, mana seems to be a part of natural existence. Scholars have found that mana filters in from the Astral Plane, and seems to seep out of the world unless contained somehow. 
Given no interference of any kind, mana will evenly spread itself across existence. However, the presence of objects, beings, flows of energy, and other such things cause mana to take up a reflection of those things, and pre-existing mana will cause newly added mana to flow like a fluid, forming currents and eddies much like water.

Using Mana

Living beings, by existing, take in mana, process it into a pure "type-less" form using biological energy, store it for a while within themselves, and slowly leak it into the environment. They can also, with effort and focus, channel the energy they store in their body in to perform otherwise impossible feats, in a process labelled "Magic". This store of mana, known as the "mana pool" or "soul", has no specific location, as it is evenly distributed across a being's material form. It is extremely difficult to access without the consent of the being it resides in.
Through unethical experimentation, the structure of the mana pool has been found to be made up of a solidified form of mana, which can be sublimated to access the significant amount of mana over and above the mana the subject can normally output. The process of accessing this mana is universally lethal to the subject.
Mana can be stored independently of any living being. Certain parts of (hopefully former) living beings are good at storing mana, such as bones or specific parts of mana-rich plants, in addition to the base component of existences spawned out of dense mana eddies (slimes and elementals) and specific mineral formations. People can also create magical devices that store and manipulate mana to produce extraordinary effects.

Mana Decay

Considering that the total density of mana has not increased a measurable amount over the (at minimum) thousands of years of existence, it is a fact that mana leaves the world somehow. The most obvious way it does so is upon the death of a living being, wherein a short time after the material body ceases to function, the mana pool disappears without a trace. It is also suspected that, given a very long time, mana will slip back through the structure of the world the same way it enters.

Manifestation

Mana is difficult to notice normally. If a being is not looking for it, they will likely sense nothing, as they are used to the sensation of mana around them, and it doesn't often change abruptly enough to be of notice. However, beings can channel a little mana into themselves to sense mana around them, typically by seeing glowing lines or blobs in the air around them.

Type

Metaphysical, Arcane