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 Soul Reapers barehanded.
The strength of Arrancars' Hierro depends on their spiritual power; Nnoitora Gilga 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 Visored 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 Shusuke 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 Visored, 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.
Darkness within ShadowEdit
In DWS Hierro is not only a arrancar technique but a Soul Reaper, Zanpakuto Spirit, and Visored technique as well as a Reigai technique.