Seithr (魔素, Maso, Magic Element), in BlazBlue is a substance that covers the planet. It can neither be seen nor smelled. In small doses, seithr is harmless to humans, but in larger quantities can carry very detrimental effects; one can become addicted and their body will deteriorate. During the Dark War, The Black Beast spread this seithr all over the world, in order to operate. Because of this, the Six Heroes taught the humans the secrets of the "Magus" in order to defeat the beast. Ars Magus was created to consume the seithr and utilize it as a source of energy.
When one uses Ars Magus, they draw seithr out of the environment to power it but there is only so much seithr in a given area. If one were to use up all of this seithr, they would have to wait a while for it to replenish. Some people can perform Ars Magus with less seithr than others. After the Dark War, a lot of humans and animals became tainted with the seithr and died off, save for the Kaka Clan. Humans were forced to move to high-altitude areas and create cities there. This is where the Hierarchical cities come from.
Seithr is derived from Seiðr, an Old Norse term for a type of sorcery or witchcraft.