Hierro

is a defensive Arrancar technique in which the user's reiryoku condenses, creating a steel-hard skin strong enough to block even a Zanpakutō. This allows users to confront some of the sword-wielding Shinigami barehanded.

Description
While this technique is exhibited by all Arrancar to some degree, the strength of an individual's Hierro is entirely proportional to the potency of their reiryoku, with Nnoitra Gilga possessing the self-proclaimed strongest Hierro amongst all generations of the Espada. However, it is unknown if such claims were true, given the fact that Nnoitra was outranked by five Espada of higher power and a former third Espada was able to injure him without complication. Although Hierro can defend against the attacks of even a released Zanpakutō, Kenpachi Zaraki discovered that he could cut Nnoitra's body in spite of it, by adapting to the latter's reiatsu.

The technique itself is believed to be exclusive to Arrancar as it has yet to be evidenced outside of such beings, including those who are capable of Hollowfication, such as the Visored. However, in his half-transformed Hollow state, Ichigo Kurosaki exhibited an apparent variation of this technique where, after enduring several injuries inflicted upon him by the Visored, he displayed an ability to regenerate his wounds with bone-like carapaces that seemed to be just as resilient as the standard Hierro, if not more so.

Trivia

 * In the English dub of the Bleach anime, Hierro was mistakenly said as Cero when Ulquiorra Cifer spoke about Ichigo being able to effortlessly cut off Yammy Llargo's arm.