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 pressure and spiritual energy 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 spirit energy.

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 unconsciously during his subsequent battle with Yhwach.

Reiatsu Colors
Different spirits have different colors of Reiatsu, and it does not depend on the person's race or power; instead, the color is determined by the owner’s personality.

Shape
Some particular individuals have a shape or manifestation of their energy whenever they release enough of it. The manifestations are one of their inner strength/energy.

Standard type
This is the most common form of Reiatsu, which have no other effect than those described above, along with its unique color and feeling. Most Hollows and Shinigami have this type, while every Quincy and Humans always have this type. Fullbringers are the only Humans to have Hollow-like powers. Yasutora Sado noticed he felt 'more at home' in Hueco Mundo, and his Reiatsu feels like a Hollow despite his species; it is still a standard type.

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

Element type
Two Shinigami have used elemental Reiatsu, even without releasing their Zanpakutō, or it being their natural Spiritual Power:

Fire & Heat: Genryūsai Shigekuni Yamamoto. Captain Yamamoto has used his burning spiritual pressure 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 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, affecting his grandma and causing her to shrink because of the sheer cold. This cold power caused others to fear him.