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1.
A sword forged from a shinigami soul and has a name by which the shinigami must learn to release it to shikai or bankai.
Ichigo's zanpakuto name is Zangetsu.
by Shinigami Daikou Kurosaki Ichi October 19, 2008
 
2.
Japanese for soul-cutting sword.

The supernatural swords that soul reapers use to exterminate Hollows and perform Honso on Wholes. Can attain two levels of power; shikai and bankai. These levels often change the look of the sword or the soul reaper. Each zanpaku-to has a unique name and call phrase.
"Rumble!! Iwahyouryu!" - anon. Sould Reaper yelling call and name of zanpaku-to.
by Verdugg the Imperialist December 20, 2008
 
3.
It is the term used to refer to the male penis.
"Girl, you hit my Zanpakutō."
by timmycolossus October 22, 2009