A zanpakutou's final form is called "Ban-Kai", which means "Full Release". It usually takes many years to master it. A bankai can be ten times more powerful than the first release, "Shi-Kai".
"Ban-Kai" is a japanese term used in "Bleach".
Ex. - Don't tell me you have forgotten that I, too, have bankai.
- Then I will have to go all-out... Ban-Kai!