In a computer file system that is organized as a hierarchy or tree, the root directory is the directory that includes all other directories. (Unlike a real tree, a tree file system has only one root!) In UNIX-based as well as in other operating systems, the root directory has no name. It is simply represented by the special character that separates directories in a file system.

In UNIX-based systems, the root directory is represented simply as:
/

Only a few special users of a shared operating system will be given the authority to access all file directories and files under the root directory. In a UNIX environment, the special user is known as an avatar.