Ars Magus is a system of power created through the combination of magic and science. It was first developed by Nine the Phantom during the Dark War. Its more advanced form is called Ars Armegis, and its strongest form are the illusive Nox Nyctores.
At some point in her life, Nine had begun researching how to combine the properties of science and magic to create a new form of energy, but she never completed her research. Soon after, the Dark War arrived, bringing the Black Beast, a being incapable of even being scratched by anything within Logic. Humanity was largely unable to learn magic, which forced Nine to resume her research in a one year span that was created by Ragna the Bloodedge. With the forced assistance of Yuuki Terumi, Nine not only managed to create Ars Magus, but had also made it combat ready - as it was made with magic and uses seithr to function, Ars exists outside of Logic and could damage the Black Beast as a result. With Ars Magus, humanity, led by the Six Heroes defeated the Black Beast. Kokonoe noted that creating Ars Magus in such a short time and then putting it on the battlefield should have been impossible. 
Ars Magus cannot be used by everyone, which has led to a widening socio-economic gap in society between those who can and cannot use it. In order for an Ars to function, it requires a Grimoire, which are regulated, stored, created, and controlled by the Novus Orbis Librarium. Nearly 100 years after the Dark War, Ars Magus is a cornerstone of society, fuelling transportation, power, climate regulators, etc.
Since seithr is required to activate Ars Magus, if there is not enough in any given area, then it will not activate. Some people are more naturally adept at using Ars than others. For example, if one person activates a healing Ars and consumes 20 units of seithr, and someone else does the exact same thing but only consumes 10 units, then the latter person has more aptitude.
A new power created by fusing the ancient power of magic with science. It was developed by the Great Magister Nine.
Armed with the power of 'Ars Magus', humanity was able to join the Six Heroes in a counterattack against the 'Black Beast.' 
- BlazBlue: Central Fiction, Story Mode, Episode 010, Chapter 082
- BlazBlue: Central Fiction, Library Mode, Weapon #022