Yggdrasil

Yggdrasil is the name of the authentication system that is currently used for authenticating user credentials for Minecraft.

Minecraft 1.6 introduced a new authentication scheme called Yggdrasil which completely replaces the previous authentication system. Mojang's other game, Scrolls, uses this method of authentication as well. Mojang has said that this authentication system should be used by everyone for custom logins, but credentials should never be collected from users. It, unlike previous versions, stores an access token instead of storing an encrypted form of the Mojang account email address and password.

The name Yggdrasil is a reference of the "World Tree" in old Nordic mythology.