The name given to Himura Kenshin during the Bakumatsu, the Meiji Revolution, in the anime Rurouni Kenshin. It means 'Battousai the Manslayer', when translated literally. 'Battousai' is taken from the word 'Battoujutsu', which is the art of drawing a katana and slashing with it all in one movement.
Kenshin retains the name throughout the Rurouni Kenshin OVAs and the original series. It is a name which strikes fear into the hearts of his enemies.
In the original series, Kenshin suffers from split personality disorder, one side is the humble wanderer Kenshin and the other is the Hitokiri Battousai Kenshin. This is sometimes referred to as 'transforming into the Battousai', a prospect which frightens Kenshin and his friends immensely.
Shinsengumi Soldier: Red hair and a cross-shaped scar!
Shinsengumi Soldier 2: It's the Hitokiri Battousai!