You unmount a volume, and eject a drive. For optical drives you can unmount the volume but the disk will still remain in the drive. Ejecting it removes the disk. For hard drives, however, for the most part the terms are pretty much interchangeable.

What is the meaning of unmount?

to dismount. 2. ( transitive) to unfix from a backing or support; remove.

What does it mean to unmount a disk?

(1) To disconnect a disk drive or optical disc from a computer. When a user selects "eject" to evacuate an optical disc from the computer, the operating system unmounts the medium. ... (2) To remove a disk or tape cartridge from the drive.