You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.vm.box
Default value: nil
Every Vagrant virtual machine is built from a base image. In Vagrant, these are distributed as “boxes.” This configuration option specifies the logical name of the box for the VM to be built against. The logical name is the name used when calling vagrant box add.