Jaki

is the malicious energy around a yōkai, sort of like a demonic aura. The stronger the demon, the stronger their jaki is. It should be noted that Jaki is not the same as Yōki, as yōki is the life force of a yōkai, whereas jaki is more to do with the darker nature or will ('yin') of a yōkai. As Tōtōsai told Inuyasha after he had gained the Dragon-Scaled ability for Tessaiga, but was experiencing backfire, jaki is a kind of poison.

Types of Jaki

 * Demonic Jaki: All demons have the potential to emanate jaki, but very evil demons have the ability to use jaki and strengthen it in order to corrupt objects and people. For example, Naraku used it to taint the Shikon no Tama and force 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 of the large amounts of evil people or creatures that go there and conglomerate, emanating a considerable amount of jaki.

Effects of Jaki
A demon with great control over their jaki is capable of corrupting others, forcing them to show the evil side of themselves, or tainting objects, and can affect the spirit and willpower of a human.

Trivia

 * The term, Jaki, is derived from jaki, another name for an  ("heavenly evil spirit"), a kind of Japanese demon (commonly depicted as a kind of small Oni), identified for its ability "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 'evil energy' or 'evil spirit': The character 邪 meaning wrong, evil, depraved, vicious, perverse,  and the character 気 meaning spirit.

Jaki