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.
Surprisingly, small amounts of seithr might have favorable effects on the world, as in the deeps of First Area, the seithr leaking from the Cauldron was neutralizing radiation and other harmful substances, clearing the air of any pollution. However, higher concentrations float around in clusters of toxic fog.
It can be categorized as a particle-type phenomenon.
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.
The seithr was present in the atmosphere before the Black Beast appeared, but since only magicians and exorcists had uses for it, the public was unaware of its existence. Later it was also used by Drive-Union in a similar manner as Ars Magus.
It is believed that all living things are made of seithr and humans can even be broken down to seithr either by the "Soul Eater" or when Drive-Unions progress to Phase Six; Acht being a prime example.
In the end, the world has been depleted of seithr due to Bang Shishigami having activated Kushinada's Lynchpin. As the exact properties of Tenjō Amanohokosaka's Soul remain unexplained, it is unknown for how long this state will remain.
Seithr is derived from Seiðr, an Old Norse term for a type of sorcery or witchcraft.