1. [Unix] The superuser account (with user name ‘root’) that ignores permission bits, user number 0 on a Unix system. The term avatar is also used.
2. The top node of the system directory structure; historically the home directory of the root user, but probably named after the root of an (inverted) tree.
3. By extension, the privileged system-maintenance login on any OS. See root mode, go root, see also wheel.