Single user mode

Single user mode is a mode in which a multiuser computer operating system boots into a single superuser. It is mainly used for maintenance of multi-user environments such as network servers. Some tasks may require exclusive access to shared resources, for example running fsck on a network share. This mode may also be used for security purposes - network services are not run, eliminating the possibility of outside interference. On some systems a lost superuser password can be changed by switching to single user mode, but not asking for the password in such circumstances is viewed as a security vulnerability.

Mac OS X

Mac OS X users can accomplish this by holding down Apple key/Command (for users of Mac OS 10.4 or up)+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).

Microsoft Windows

Microsoft Windows provides Recovery Console. It is different from single user modes in other operating systems because it is independent of the maintained operating system.

=Unix family=
Unix-like operating systems provide single user mode functionality through runlevels. Runlevels are usually changed using the init command, runlevel 1 or S will boot into single user mode.