In the original American SNES version of Final Fantasy IV, Odin's attack was called Z-Sword. This was most likely because the developers could not fit the entire word "Zantetsuken" in the available text box. In Final Fantasy VIII, Final Fantasy IX, Final Fantasy XII: Revenant Wings, and Crisis Core -Final Fantasy VII-, the weapon/attack is called Zantetsuken. The sword is curved with a unique shape to it, as seen in its most popular form in Final Fantasy VIII, where it looks more like an axe at first glance. This sword is used to instantly kill the enemy it hits. In Final Fantasy VII, it's also known as the "Steel Bladed Sword", which is a mistranslation from the Japanese "Zantetsuken". The accurate translation is "Iron-Cutting Sword".
The Zantetsuken also appears as a usable weapon in Final Fantasy VI. It has the ability to randomly dispatch an enemy, and is also effective against the undead. It was called the Scimitar in the original SNES release. The summon Attack of the Esper Odin is also called Zantetsuken, the one of Raiden is called Shin-Zantetsuken.
The Slash-All Materia can be upgraded to Flash, which instantly kills all enemies with a sword stroke.
In Final Fantasy X-2, the Samurai Dressphere has two abilities based on this weapon. They are Zantetsu, which has a chance to instantly kill a single target (Zanmato style) and Shin-Zantetsu which does the same, but targets all enemies and attempts to inflict the Instant Death status.
Zantetsuken is the name of Odin's ultimate attack when he's battled in the last chamber of Einherjar, which he uses at 10% health. This ability will instantly kill any players in range, unless they /kneel or /heal, which can prevent instant death (but not damage).
Gilgamesh holds a replica of this legendary sword.
In Ring of Fates, the Zantetsu appears as a weapon for Clavats. The description reads "A sword that cuts through anything. Passed down from abroad a long time ago."