Bankai (卍解, Final Release), originally known as Shinuchi (真打, True Strike), is the second and final form of a Shinigami's Zanpakutō. It is the ultimate technique of Captain-class Shinigami. Those who achieve it are said to be in possession of an "evolved Zanpakutō" (進化した斬魄刀, shinkashita Zanpakutō).
To achieve Bankai, a Shinigami must typically train upward of ten years to both subjugate and materialize their Zanpakutō spirit in the real world. Afterwards, they must complete additional training to fully master their Bankai before it is useable in combat. The spirit learns Bankai at the same moment as its Shinigami wielder, but it can withhold power from its Shinigami if it deems them to be unworthy. Recently obtained Bankai can continue to evolve as their Shinigami grows until the Zanpakutō is completely mastered.
Bankai typically create a large effect, though this is not always the case, such as Ichigo Kurosaki's Tensa Zangetsu. They also typically increase their wielder's strength from anywhere between five and ten times their base level. A Bankai's form and power will only disappear if a Shinigami wills it or if they are on the verge of death.
Breaking: A broken Bankai can never be naturally returned to its former state. Even if one cosmetically fixes their Bankai, the resulting power loss can never be amended. A few Bankai are an exception to this rule, such as Sajin Komamura and Raian Getsueikirite's Bankai. Broken Bankai that are not finished evolving can seemingly be fixed by completing them, such as in the case of Renji Abarai and Ichigo Kurosaki.
Hollowfied Bankai (虚(ホロウ)化卍解, Horōka Bankai): By introducing a Hollow's Reiatsu to a Shinigami Bankai, such as with the inventions of Kisuke Urahara or Sadaharu Urahara, a Shinigami's Bankai can temporarily Hollowfy, granting them immunity from Quincy techniques and, in some cases, Hollow powers.