Jaki

Jyaki (邪気, (じゃき,) evil energy), this is commonly 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
Demons Jyaki - all demons has the potential to emanate jyaki, but really evil demons has 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.

Hell Jyaki - it is implied in latter events that the user of the Meidou Zangetsuha must be strong enough to withstand the jyaki form 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.