Reiryoku

Reiryoku (霊力, Spiritual Power) is a power aligned with the spiritual sensitivity and willpower of the user. It is used by Shinigami and other spiritual beings to provide power for their various abilities.

Overview
Every spiritual being and every Human has a certain amount of Reiryoku. If this amount is higher than a certain degree, it grants the person superhuman abilities. Those with such an above-average level of Reiryoku are very rare among living Humans. It seems to be that the most basic ability that Humans with a higher level of Reiryoku have is the ability to see ghosts: Pluses, Shinigami and Hollow alike. Shinigami are essentially souls in Soul Society with very high levels of Reiryoku who receive special training to utilize their powers. Hollows and other spirits use their Reiryoku to catch their prey.

Sensing Reiryoku
Another basic ability that beings with higher-than-average levels of Reiryoku possess is the ability to sense other spiritual beings without relying on their five basic senses. This is possible because every spiritual being emits a certain amount of his/her own Reiryoku, which can be sensed. This ability apparently requires some innate skill, but can be increased with training.

Every spiritual being has a unique pattern to his/her emitted Reiryoku. One can recognize the pattern of their friends and acquaintances after some time, being able to sense their presence and their physical and mental state from relatively great distances. Sudden changes in Reiryoku pattern can mean that the person in question is in great stress (i.e. fighting) or has received a grave wound.

Different types of spiritual beings have greatly different patterns, the Reiryoku pattern of Hollows for example is easily distinguishable from that of Pluses or a Shinigami. The Visored are a special case, because when their Hollow mask is donned, their pattern of Reiryoku resembles that of a Hollow, rather than that of a Shinigami.

Controlling Reiryoku
The aforementioned emission of Spiritual Energy is a common and natural phenomenon, but a spiritual being can also control the degree of this emission, this usually requires a certain amount of skill and training. By completely blocking his/her Reiryoku "pouring out", one can completely hide his/her presence from other spiritual beings and spiritually aware humans. On the other hand, amplifying this emission is known as Reiatsu (霊圧, Spiritual Pressure), is considered a show of force, a means of intimidation or can even be an attack in itself. It can indeed paralyze opponents and cause direct damage to the surrounding individuals and inanimate objects alike.

Aside from controlling the amount of emitted Reiryoku, most spiritual beings and some humans are capable of performing many superhuman feats with the help of their Reiryoku. These feats usually require a certain level of concentration, and they tend to tire the person after some time, or after performing some of them in quick succession. It is also known, that after excessive usage of Reiryoku, Shinigami need food to replenish their strength, while on the other hand, the regular souls that dwell in Soul Society that do not use their Reiryoku do not need to eat.
 * Aside from Spiritual Pressure, Shinigami and Visored generally use their Reiryoku to
 * cast Kidō spells
 * increase their speed and sharpness of their movements (Hohō)
 * use the special abilities of their Zanpakuto
 * increase their physical strength and power
 * Hollows and Arrancar have other abilities, namely Cero and Bala, that are condensed Reiryoku fired towards the opponent. In the case of the former, it is done in a laser-like manner; in the latter case, the fired Reiryoku is bullet-like. Hollows and Arrancar can also increase their mobility like the Shinigami above, and the usage of their Zanpakutō also relies on the usage of Reiryoku, albeit in a different manner. Hierro is also a move that uses an Arrancar's Reiryoku for defense.
 * Quincy can also increase their mobility, and form a weapon (usually a bow) with their Reiryoku. In their usage of Reiryoku, however, there is a fundamental difference from spiritual beings – they primarily rely on collecting reishi from the environment, and use this collected reishi to fight. They can even collect and store Reiryoku in various special items.