A disk is a device that holds data. Some common types of disks are hard drives, thumb drives and flash drives, zip disks, CDs and DVDs.
For all types of disks, saying that a volume or disk is mounted is saying that a disk is in a usable state. On a Macintosh, inserting a disk will cause the computer to automatically mount the disk, and ejecting the disk will cause the computer to unmount the disk.