High-Speed Regeneration

is a supplementary ability possessed by many Hollows, some Arrancar, and even Shinigami who have undergone Hollowfication.

Description
High-Speed Regeneration allows Hollows to recover from any damage they sustain by allowing them to heal at a drastically increased rate, closing the wounds on their body and growing back any lost limbs within extremely short periods of time. Despite the many benefits 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 instantly. While this ability to regenerate is indeed powerful, 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 at once, it can be destroyed nonetheless.

Trivia

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