Single-user mode

Single User Mode is a mode in which a multiuser computer operating system boots into a single superuser, allowing the end user to troubleshoot the system.

Mac OS X

Mac OS X users can accomplish this by holding down ⌘S after powering the system. The user may be required to enter a password set in the firmware. Single User Mode is different from a Safe Mode boot in that the system goes directly to the console instead of starting up the core elements of Mac OS X (items in /System/Library/, ignoring /Library/, ~/Library/, et al.). From there users are encouraged by a prompt to run fsck or other command line utilities as needed (or installed).