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 jyaki is. It should be noted that Jyaki is not the same as Yōki, as yōki is the demonic energy.

Types of Jyaki
Demonic Jyaki - all demons have the potential to emanate jyaki, but really evil demons have the ability to use jyaki and strengthen it in order to corrupt objects and people. Such as Naraku tainting the Shikon Jewel and forcing Inuyasha to transform into a full demon. Jyaki is also present in Miasma, if that demon can produce a miasma.

Hell Jyaki - it is implied in latter events that the user of the Meidō Zangetsuha must be strong enough to withstand the jyaki from hell, this could be because the large amounts of evil people that go there and conglomerate to emanate a considering amount of jyaki.

Effects of Jyaki
As mentioned above the jyaki is the evil energy, a demon with great control over the jyaki 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, Jyaki, 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 Jyaki, (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.