The Shikai (始解; initial release) is the first "upgraded" form available to a Zanpakutō. To activate it, the Shinigami needs to learn the name of their Zanpakutō (one notable exception is Kenpachi Zaraki who was able to achieve Shikai without learning the name of his Zanpakutō). This is not as easy as simply picking a name, as the living spirit of the Zanpakutō already has its own name. Therefore, the Shinigami must be able to communicate with their Zanpakutō effectively, which requires being able to speak to the spirit within its world. Achievement of the Shikai is a mark of control of a Zanpakutō, and it appears to be a requirement for advancement within the Shinigami ranks, as most seated officers and presumably all Lieutenants are capable of Shikai.
After first learning the Zanpakutō's name, the Shikai can be performed at will simply by speaking a command followed by the Zanpakutō's name. The commands vary between users and range from a simple imperative verb to a short poem. They often relate to the Zanpakutō's signature ability, or hint at the nature of its spirit. This step may be bypassed by expert Shinigami who have learned how to use their Bankai, although this has only been demonstrated once, by Renji Abarai. In rare cases, a Zanpakutō, once released, will remain in that state indefinitely. Such Zanpakutō is described as a full-time released form type, such as Ichigo's.
Once the Shikai is activated, the Zanpakutō will change form to access whatever special abilities it is capable of. They usually remain in the form of a sword (with multiple exceptions, such as Soifon wielding a stinger, Ikkaku Madarame wielding a spear, Kaien Shiba wielding a trident, Marechiyo Ōmaeda wielding a flail and Shūhei Hisagi wielding two connected dual scythes), but the type of sword differs radically among various Zanpakutō. The special abilities of the Zanpakutō, like the Zanpakutō itself, have specific names that usually must be announced to fully activate the effect. As Ichigo's Zanpakutō, Zangetsu, explains, knowing the name of an attack and not knowing it makes a difference in the amount of power that attack has.
Although shikai exists for zanpakutō wielded by European shinigami, no actual name for the release has been mentioned. It was usually regarded as a "release."