Mana

Mana (魔力) is a form of energy that exists in Mushoku Tensei. It is the source of all magic and is comparable to energy in our world. All living beings (human and non-human, including monsters and plants) have mana within their bodies. Mana exists in almost every aspect of the Six-Faced World. Beings, such as Nanahoshi, are unable to hold mana within their bodies since they have tripped into the world.

Flow
Like blood, Mana circulates throughout the body of every living being.

Because of the internal constant Mana flow (which, like Disturb Magic, disrupts the formation of other flows), it is impossible to materialize magic inside the body. Every living being in the Six-Faced World has a similar protection.

Magic
Magic is the manipulation of mana. It can be classified into the following: Attack magic (basic elements), Healing magic (buff/debuff), Summoning. Furthermore, unorthodox magic that is difficult to classify under the three main categories are referred to as unique magic (or original magic in cases of innovation, such as Electric).

It should be noted that while magic does seem to follow the law of conservation of energy, it demonstrably doesn't. Otherwise, Rudeus' mass would be equivalent to that of his body, plus the total mass that can be generated with his mana.