Reiatsu

is the physical force/pressure that a person’s Reiryoku creates when released. Most Shinigami, Arrancar, Quincy and Bounts can manipulate the release of their Reiatsu.

Reiatsu vs. Reiryoku
The difference between Spiritual Power (Reiryoku) and Spiritual Pressure (Reiatsu) is simple: Reiryoku is the amount of energy a being has stored within their body or soul, whereas Reiatsu is the pressure that a person's Reiryoku exerts. In other words, Reiryoku is potential while Reiatsu is energy in use and can be sensed by other spiritually aware beings. In general, those with high levels of Reiryoku will often have the highest levels of Reiatsu also. A skilled warrior can overcome a person possessing greater Reiryoku by possessing greater Reiatsu - this is achieved by having greater control over their own Spiritual Power.

Reiatsu Sensing
The senses by which one feels and pick up on Reiatsu are known as or. Every person that has Reiatsu subconsciously uses both their sense of vision and their Reikaku at the same time in order to see. When concentrating in the midst of battle, Reikaku takes over an overwhelming amount of perception; in other words, the subconscious begins to stop seeing with "eyes".

Reiatsu Vents
All Shinigami have vents located at their wrists that vent out their Reiatsu. If they are blocked off, then the Shinigami in question will be incinerated by their own Reiatsu from within.

Reiatsu Release
All Reiatsu released by individuals eventually returns to them, but Reiatsu released in explosive bursts also engulfs energy in its surroundings in the process; when Ichigo Kurosaki did so while attempting to escape Quilge Opie's The Jail, Quilge's Quincy Reiatsu seeped into the depths of Ichigo's soul and awakened the Quincy aspect of his power within it, causing Ichigo to begin using Blut Vene subconsciously during his subsequent battle with Yhwach.

Spirit Class
The density of Reiatsu within the Reishi of a given individual determines their, ranging from 1 to 20 starting with the densest Reishi and consequently most powerful individuals; Lieutenants of the Gotei 13 are of rank 4 or 5, while Captains are of rank 1, 2, or 3. However, the Reishi comprising the bodies of those with a Spirit Class of rank 3 or higher are too dense to break down and return to the soil of Soul Society after death as the bodies of Souls normally do, and since it would be too risky to leave such strong Reishi remaining indefinitely in Soul Society, the Konsō Reisai ceremony is performed to cast captains who had such Spirit Class at the time of their deaths into Hell. When Kaien Shiba entered the Shin'ō Academy, his Spirit Class was recorded at rank 6, which was comparable in strength to a Lieutenant.

Reiatsu Colors
Different beings have different colors of Reiatsu, and it is not dependent on the individual's race or power. Certain combatants, like Ichigo Kurosaki, also have the color of their Reiatsu change over time or while using certain powers.

Shape
Certain individuals can generate a shape or manifestation of their Spiritual Power whenever they release enough of it; the manifestations are one of their inner strength.

Standard type
This is the most common form of Reiatsu, which has no effects other than those described above, along with its unique color and feeling. Most Hollows and Shinigami have this type, while Quincy and Humans always have this type. Notably, Fullbringers are Humans who have Hollow-like powers, and Yasutora Sado noted he felt "at home" in Hueco Mundo because his Reiatsu feels like that of a Hollow despite his species, but their Reiatsu is still a standard type.

Dual type
All Visored and Arrancar have this specific Reiatsu; Visored feel more like Shinigami, while Arrancar feel more like Hollows. When Ichigo's inner Hollow started to take over his body, Orihime Inoue could sense it and was frightened by how heavy and dark it was. The Visored also feel more like Hollows when they release their powers. Arrancar seal their Hollow abilities and become as close to their original form as can be when they enter their Resurrección, explaining this phenomenon.

Element type
Two Shinigami have used elemental Reiatsu, even without releasing their Zanpakutō, with it being their natural power:

Fire & Heat: Genryūsai Shigekuni Yamamoto. Captain-Commander Yamamoto has used his burning Reiatsu to heat up his sealed Zanpakutō, causing it to glow dark orange like a recently forged sword. He has also displayed a large amount of flames emanating from his body before releasing his Shikai, which is needed for his fire-based techniques.

Ice & Cold: Tōshirō Hitsugaya. When Tōshirō was a child, he had trouble controlling his power. In his sleep, it would go out of control, freezing his grandma with the sheer cold. This cold power caused others to fear him.

Trivia

 * Sōsuke Aizen once theorized that one could gain an immense boost to their physical prowess if they were to somehow discard all of their Reiatsu.

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Reiatsu Reiatsu