Remember, media is "being used" if a shell or an application is accessing any of its files or directories. If you are not sure whether you have found all users of a DVD (for example, a shell hidden behind a desktop tool might be accessing it), use the fuser command. See How to Determine If Removable Media Is Still in Use.
# eject media
For example, for a DVD, you would do the following:
# eject cdrom
For example, for a USB memory stick, you would do the following:
# eject rmdisk0