High-Speed Regeneration

is a supplementary ability possessed by many Hollows, as well as some Shinigami that have undergone Hollowfication and Arrancar.

Description
High-Speed Regeneration allows Hollows to recover from any damage that they sustain by causing them to heal at a drastically increased rate, closing the wounds on their body and causing any limbs that they have lost to grow back within extremely short periods of time. Despite the many benefits that it provides, most Hollows choose to abandon the ability to regenerate when they become Arrancar, in exchange for a greater amount of power. Regardless, when Arrancar use their Resurrección, their wounds heal away instantly. While High-Speed Regeneration is indeed a powerful ability, it is not without its flaws. Despite the extent to which it can heal the body, internal organs such as the brain cannot be regenerated; if such irreparable damage is sustained, the rest of the body can continue to regenerate, though to no avail. In addition, if the body is damaged before it has the chance to heal, or if enough damage is inflicted on it at once, then it can be destroyed.

Trivia

 * After fusing with the Hōgyoku, Sōsuke Aizen attained a similar ability, though he differentiated it from the regeneration that Hollows possess.