umount does the opposite of mount: it makes a disk partition unavailable via the filesystem. For instance, if you’ve mounted a CD-ROM disc, you can’t eject it until it’s umounted:

$ umount /media/cdrom

Always unmount a removable medium before ejecting it or you risk damage to its filesystem. To unmount all mounted devices:

# umount -a

Don’t unmount a filesystem that’s in use; in fact, the umount command will refuse to do so for safety reasons.