Shōki, (Shouki, (瘴気, (しょうき,) lit. translation, "Swamp Gas." English Dub; "Miasma," ("Bad Air.")
Miasma is highly-corrosive and poisonous gaseous mist, a purple, smoke-like form, used by many demons as a means of both attack and/or defense. The Jaki of any Yōkai who can generate a miasma is also present in their miasma, like as in a unique signature. The more potent one's jaki, the more powerfully-corrosive their shōki/miasma becomes.
Naraku makes a widespread use of miasma, even using it as a means of delivery of his Jaki to infect people.