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On many computer operating systems, superuser, or root, is the term used for the special user account that is controlled by the system administrator.

Many older operating systems on computers intended for personal and home use, including MS-DOS and Windows 9x, are not multi-user operating systems, and there is no distinguished superuser account. Effectively any user of the system has administrator privileges. Separation of administrative privileges from normal user privileges makes an operating system more resistant to viruses and other malicious software, and the lack of this separation in these operating systems has been cited as one major reason why these operating systems are insecure.

In Unix-style computer operating systems, root is the conventional name of the user who has all rights or permissions in all modes (single- or multi-user). Alternative names include baron in BeOS and avatar on some Unix variants. BSD often provides a toor ("root" backwards) account in addition to a root account. The root user can do many things an ordinary user cannot, such as changing the ownership of files and binding to ports numbered below 1024.

It is never good practice for anyone to use root as their normal user account because simple typographical errors in entering commands can cause major damage to the system. It is recommended to create a normal user account instead and then use the su command to switch when necessary. Some use the sudo utility instead, which allows a measure of graduated access.

Many operating systems, such as Mac OS X and Linux distributions, allow administrator accounts which provide greater access while shielding the user from most of the pitfalls of full root access. In some cases, the root account is disabled by default, and must be specifically enabled. In a few cases, such as Plan 9, there is no superuser at all.

Software defects which allow a user to "gain root" (to execute with superuser privileges code supplied by that user) are a major security issue, and the fixing of such software is a major part of maintaining a secure system. One common way of gaining root is to cause a buffer overflow in a program already running with superuser privileges. This is often subsided in modern operating systems by running critical services, such as httpd, under a unique limited account. A related term is rootkit, referring to using root privileges to conceal various data from the system administrator.

In Windows NT and later systems derived from it (Windows 2000, Windows XP, Windows Server 2003 and Windows Vista), there may or may not be a superuser. By default, there is a superuser named Administrator, although it is not an exact analogue of the Unix root superuser account. Administrator does not have all the privileges of root because some superuser privileges are assigned to the Local System account in Windows NT.

To run a program as a superuser in Windows XP or Windows Vista , use the command runas. See Microsoft's documentation for runas for more details.

• Power user
• nobody

• root Definition - by The Linux Information Project (LINFO)
• LINUX: Rute User's Tutorial and Exposition
• Discussion on origin of Charlie Root at pipermail