Jaki

, this is commonly referred to as the malicious energy around a yōkai, sort of like a demonic aura. The stronger the demon, the stronger his jaki is. It should be noted that Jaki is not the same as Yōki, as yōki is the demonic energy while jaki is the demonic aura.

Types of Jaki

 * Demonic Jaki - all demons have the potential to emanate jaki, but really evil demons have the ability to use jaki and strengthen it in order to corrupt objects and people. Such as Naraku tainting the Shikon no Tama and forcing Inuyasha to transform into a full demon. Jaki is also present in Shōki, if that demon can produce a miasma.
 * Hell Jaki - it is implied in later events that the user of the Meidō Zangetsuha must be strong enough to withstand the jaki from hell, this could be because the large amounts of evil people that go there and conglomerate to emanate a considering amount of jaki.

Effects of Jaki
As mentioned above the jaki is the evil energy, a demon with great control over the jaki is capable of corrupting, forcing targets to show their evil parts, tainting objects and can affect the spirit and willpower of a human.

Trivia

 * The term, Jaki, is derived from jaki, another name for an Amanojaku, ("heavenly evil spirit,") a kind of Japanese demon, (commonly depicted as a kind of small Oni,) identified for its ability to "to provoke a person's darkest desires out of them, and thus instigate them into perpetrating wicked deeds."
 * The characters for Jaki, (邪気,) literally translate as an 'evil energy' or 'evil spirit': The character, 邪, meaning; wrong, evil, depraved, vicious, perverse, (an accurate description of Naraku,) and the character, 気, meaning; spirit.