On Unix systems this is done by associating the FileSystem being mounted with a the MountPoint, ie an actual directory in an already mounted FileSystem. The "root" file system is mounted on the root directory, /, early in the boot sequence.
Apparently derived from the physical sense of "mount" meaning "attach", as in "head-mounted display", or "set up", as in "AlwaysMountaScratchMonkey".
See also FileSystemHierarchy.
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