The only directory within a filesystem which is not a subdirectory of any other directory. In other words, the "highest-up" member of the directory tree.
Under Unix, it is often referred to as "slash" in spoken dialogue, since its full pathname is just a single /. Under MS-DOS, the root directory of a particular drive is referred to instead by a backslash, for example: C:\. Under Windows, of course, the user can remain blissfully unaware that any such concept exists--the technical term for "root directory" amongst WIMPs is "the little clicky thing that all the other little pictures pop out of when you click on it, except when clicking on stuff makes the screen turn blue and you have to reboot and start working on your Gantt chart again".