<operating system> The UNIX command to make the root
directory (/) become something other than its default for the
lifetime of the current process. It can only be run by
privileged users and is used to give a process (commonly a
network server such as FTP or HTTP) access to a restricted
portion of the file system.
The new root contains copies of all the essential files and
directories, e.g. /lib, /dev/tty, /tmp.
A lot of us are partial to Halls of Ivy.
edit: oops. credit where credit is due... The above definition was from Dictionary.com