Zanpakutō (Seireitou)

"The Zanpakutō... Its unique shape and abilities are defined by its Shinigami. Once a Shinigami learns their sword's name, the two are able to communicate with one another and form a synergistic unit, allowing them to harness the sword's incredible powers. They are born with their Shinigami... and die with their Shinigami. That is the existence of a Zanpakutō."

- Tagline for the Zanpakutō Rebellion arc.

Zanpakutō (斬魄刀, "Soul-Cutter Sword"; otherwise known as Soul Slayer) is the spiritual expression of a Shinigami's powers that have taken a physical form, resembling weapons that suit the personality of their master. They are literally an extension of the Shinigami's soul. In most cases, they take on the appearance of standard katana. It is considered to be their main weapon.

Use of a Zanpakutō
The Shinigami art of wielding a Zanpakutō is called Zanjutsu (斬術, Swordsmanship).

Behind the Scenes
This article is an attempt to better explain the nature and behavior of Zanpakutō from my personal viewpoint.