Yōki

is the energy and power utilized by yōkai; a demon's yōki acts as the source of their power. Demons have been shown to manifest their yōki in the form of energy attacks, channeling it through their bodies to increase their physical aspects, such as Inuyasha channeling his into his claws, or their weapons, such as Inuyasha and Sesshōmaru channeling their yōki into their sword to unleash powerful attacks, and by releasing it in the form of their jaki for varying effects. The energy can also take the form of certain elements such as fire and lightning. A demon's yōki also becomes stronger the more they commit evil deeds, and or being born from two pairs of demon's with considerable power such as daiyōkai.

Yōki is also known to be very dangerous, able to corrupt or outright destroy weaker beings.

Physical augmentation
Demons are able to use their yōki to greatly increase their physical prowess. An example this includes Inuyasha's Sankon Tessō to increase the strength and attack range of his claws.

Constructs
In some cases yōki can be used to create certain constructs. Examples of this include Inuyasha's Hijin Kessō, Sesshōmaru's Light Whip, and Naraku's barrier.

Elements
A demon's yōki can be channeled into the form of various elements. An example includes Sesshōmaru being able to channel his yōki into lightning through his Tōkijin to unleash his Sōryūha. In some cases for those already possessing elemental abilities, their yōki can be used to make them more potent, much like how Naraku is able to channel his jaki through his miasma.

Kenatsu
is an ability mostly possessed by yōkai swords. When yōki is channeled into the sword, the user is able to strike their enemies without the blade making physical contact.
 * Slashes: The most common form of Kenatsu in which users of yōkai swords project the yōki in the form of slashes.
 * Needles: This was a special case for Sesshōmaru when he wielded Tōkijin. He was able to project powerful energy needless when he needed to dispose of lesser demons.
 * Yōki Wave: When users become proficient enough with Kenatsu they are able to unleash it in the form of devastating waves of demonic energy, such as the Kaze no Kizu, to slay countless demons.

Ōgi
For some users, it serves at the basis for their ultimate attacks, allowing them dispatch of multiple enemies and powerful opponents.

Yōketsu
is the visual manifestation of demonic energy in the form of a whirlpool that appears behind a yōkai and is visible to just a few. This vortex can be cut and its effect is an instant-kill upon the demon whose yōketsu is being cut; Inuyasha's Dragon-scaled Tessaiga had this ability.

Trivia

 * Yōki is also referred to as Yōryoku (妖力, ようりょく (lit. "demonic power, energy and/or strength")), i.e. in Chapter 477.

Yōki 妖氣