A katana with a 'reverse' blade; meaning the traditional shape edge is dull and hard, and the flat back has the shape edges. This way, the sword delivers bliunt damage instead of cutting damage, ut the way it is crafted it is still VERY lethal by causing high, concentrated damage onto one spot.
The kind of sword used by Himura Kenshin in the manga known as Rurouni Kenshin.
And yes, I understand is makes NO FUCKIGN SENSE AT ALL, but you can eventually get the concept of it.
(BTW, thnx to The Dork Knight at the Anime and manga Social board from GameFAQS.com for explaining it to be originally)
A variation of the standard Japanese katana in which the edge is crafted on the inside of the blade curve, whereas the flat side is placed on the outside of the curve. This blade, known as a "reverse-blade sword," is used in the same manner as a katana, but the user is not required to kill when wielding the sakabato. Strikes are made with the flat side of the blade, or with the hilt. Sakabato began as fictional weapons, originating in the manga Rurouni Kenshin, in which the main character, Himura Kenshin, wields a sakabato to uphold his vow never to kill, yet still protect his friends with his sword.
Now, Himura Kenshin is rurouni, a wanderer, and carries a reverse-edged sakabato blade, vowing to never kill another soul.