The most basic form of Bankai, which is 5 to 10 times stronger than a shikai, which is 5 times stronger than a Zhikai.
In other words, Zhikai is teh failz.
SHERRIE: "OH MY GAWD HEZ GONNA RELEASE BANKAI."
Zhikai: "ZHI... KAI"
Randall: "...........Teh failz."