mount
To make a file system available for access.
Unix does this by associating the file system with a directory (the "mount point") within a currently mounted file system. The "root" file system is mounted on the root directory, "/" early in the boot sequence. "mount" is also the Unix command to do this, "unmount" breaks the association.
E.g., "mount attaches a named file system to the file system hierarchy at the pathname location directory [...]" -- Unix manual page mount(8).
File systems are usually mounted either at boot time under control of /etc/rc (or one of its subfiles) or on demand by an automounter daemon.
Other operating systems such as VMS and DOS mount file systems as separate directory hierarchies without any common ancestor or root directory.
Apparently derived from the physical sense of "mount" meaning "attach", as in "head-mounted display", or "set up", as in "always mount a scratch monkey, etc."
Unix manual page: mount(8).