Zanpakutou

Your zanpakutou (zanpakuto, zanpakutō are other common spellings; it is also called a 'soul cutter' in some translations of the anime or manga) is your main weapon throughout the game. It has three states: normal (unreleased), shikai (first or initial release), and bankai (final release). Each of the stages are marked by a certain appearance, progressively changing forms as shikai and bankai released. Furthermore, the release of a shikai increases the Shinigami's power, and the release of a bankai increases the power of the Shinigami by a factor of five to ten.

Feel free to read the Wikipedia article on the zanpakutou for more information.

http://en.wikipedia.org/wiki/Zanpakutou