Reiatsu

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

Overview
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. When two spiritual beings collide, the side with weaker spiritual pressure will get hurt. Reiatsu at its most basic level can instill fear in others when it is very high (in comparison to the one feeling it). Willpower is the simplest way to fight this effect. High level of spiritual power can cause those of lower power to be stunned by simply being in the presence of a high level spiritual being. When a Spiritual being of high reiatsu chooses to they can paralyze an opponent just by focusing pressure through their stare or presence. In high enough quantities, spiritual pressure can be lethal to others.