Hierro (Iero; spanish meaning for "Iron", japanese meaning for "Steel Skin") is an Arrancar technique in which the user's spiritual power condenses, creating a steel-hard skin strong enough to block a Zanpakuto. This also allows them to fight some of the sword-wielding Soul Reapers barehanded.
The strength of Arrancars' Hierro depends on their spiritual power; Nnoitra 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 Zanpakuto, Kenpachi Zaraki discovered that he could cut Nnoitra's by adapting to the latter's Spiritual Pressure.
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.