Shikai is an ability of a zanpakutou (a type of weapon in Bleach, often a sword), of transforming itself into something of a different appearance while "unlocking" special powers, less powerful than a bankai.
For example, when Sode no Shirayuki (the zanpakutou of Kuchiki Rukia) is in shikai mode, it turns from a common katana into a blue sword with ice-powers.
For the artist formerly tagged here, see shikai_(iesiki_56).
The following tags are aliased to this tag: shikai_(bleach) (learn more).