Hierro

is an Arrancar technique in which the user's reiryoku condenses, creating a steel-hard skin strong enough to block a zanpakutō. This also allows them to fight some of the sword-wielding Shinigami barehanded.

The strength of an Arrancar's Hierro depends on their spiritual power; Nnoitra Jiruga claimed to have the strongest Hierro. But it is unknown if Nnoitra's claims are true, given the fact that he's outranked by four Espada of higher power, and a former Espada was able to injure him without reciprocation. While Hierro can block even released Zanpakutō, Kenpachi Zaraki discovered that he could cut Nnoitra's by adapting to the latter's Reiatsu.

The technique is exclusive to Arrancar, and it has yet to be seen if the Vizard are capable of using Hierro during their Hollowfication. Shinigami have been shown using reiatsu to block or lessen the blows of Zanpakutō, such as when Ichigo Kurosaki used his reiatsu to momentarily stun Shūsuke Amagai during their battle. Also, when Ichigo and Kenpachi Zaraki first fought, Ichigo failed to injure Kenpachi even after he offered Ichigo an opportunity to cut him. However, this ability cannot be done to the same extent as this Arrancar technique.

Ichigo, in his half-transformed Hollow state, exhibited a variation of this technique where, after enduring several injuries inflicted upon him by the Vizard, displayed an ability to regenerate his wounds with a bone-like carapaces that appears to be just as resistant as the standard Hierro, if not more so.

Trivia

 * Although all the Arrancar possess Hierro, there is a possibility that some don't benefit from it as much as others do. The Octava Espada Szayel Aporro Granz was shown being able to bleed from even the slightest attacks, an occurrence that wouldn't have affected any of the other Arrancar.
 * In the English dub of the Bleach anime, Hierro was mistakenly said as Cero when Ulquiorra spoke about Ichigo effortlessly cutting off Yammy's arm.
 * Nnoitra Jiruga claims that his Hierro is the strongest of all the Arrancar. However, the truth of this matter is presently unknown.