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Definition: DMG

(Disk iMaGe) The file format used in the Mac for distributing software. Mac install packages appear as a virtual disk drive on the Mac. When the DMG file icon is double clicked, the virtual drive is "mounted" on the desktop. The DMG format harks back to the days when the Mac was a floppy-only machine. DMGs were used to make multiple floppy disks without having to insert the original floppy again for each subsequent copy. See disk image.

Mounting a DMG
This Toast software installation downloads from the Internet as a DMG file. When double clicked, it is "mounted," and the dialog box opens to start the installation by dragging the icon on the left to the icon on the right.