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 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. It is unknown if Nnoitra's claims are true as four Espada are of higher rank and 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, though it is yet to be seen if Vizard are capable of creating Hierro during their Hollowification. Shinigami have been shown using reiatsu to block or lessen the blows of zanpakutō, like 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 him after Kenpachi himself offered Ichigo an opportunity to cut him. However, this ability cannot be done nearly to the same extent as the Arrancar's 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, his wounds immediately regenerated into bone-like carapaces that appear to be just as resistant as the standard Hierro, if not moreso.

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.