Hierro

is an Arrancar technique in which the user's Reiryoku condenses, creating a steel-hard skin strong enough to block a Zanpakutō. 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. For example, when Ichigo Kurosaki used his reiatsu to momentarily stun Shūsuke Amagai. Also, when Ichigo and Kenpachi Zaraki first fought, Ichigo couldn't land a single scratch on him. It wasn't until the apparition of Zangetsu appeared that he begin to put more power into his attacks and regain his resolve that he managed fight on even grounds with Kenpachi. 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 more.

Trivia

 * Although all the Arrancar have the feature of Hierro, there is a good possibility that some don't benefit from it as much as the others do. The Octava Espada, Szayel Aporro Granz was shown being able to bleed from even the slightest attacks, an occurance that wouldn't have affected any of the other Arrancar.
 * Hierro was mistakenly said as Cero when Ulquiorra spoke about Ichigo effortlessly cutting off Yammy's arm (In the english dub).