Reishi

Reishi (meaning "Spirit Particles"), sometimes called Spiritrons, are the fundamental particles which make up all spiritual things. This includes the spiritual bodies of Hollows and Soul Reapers, and the two planes of existence, Hueco Mundo and the Soul Society in their entirety. Reishi is invisible to mortals; they can, however, still interact with normal matter and produce physical effects. It is noted, however, that reishi and atoms do not mix well.

There is a certain amount of reishi in the World of the Living, too. Free reishi tends to be found in places closer to nature.

Reishi in the atmosphere is also able to be controlled freely, allowing a person to gather it under their feet so that they can stand in air. Reishi forms the basis for the following techniques:
 * Flashstep: The method in which Soul Reapers use to travel faster than the eye can follow.