In Unix style computer operating systems, root is the name of the user who has all rights on permissions in all modes (single or multi user). Normally this is the administrator's account. The user root can do many things an ordinary user cannot, such as changing the ownership of files and binding to a server port

If a person 'has root access' it means that they are able to act as the administrator of that computer.

Additionally it refers to the base of the file system's directory structure. See Root directory.

Furthermore, in a tree data type, the root is the lowest node in the tree.