DMG meaning in Computing ?

A disk image, in computing, is a computer file containing the contents and structure of a disk volume or of an entire data storage device, such as a hard disk drive, tape drive, floppy disk, optical disc, or USB flash drive. A disk image is usually made by creating a sector-by-sector copy of the source medium, thereby perfectly replicating the structure and contents of a storage device independent of the file system. Depending on the disk image format, a disk image may span one or more computer files.

The file format may be an open standard, such as the ISO image format for optical disc images, or a disk image may be unique to a particular software application.

The size of a disk image can be large because it contains the contents of an entire disk. To reduce storage requirements, if an imaging utility is filesystem-aware it can omit copying unused space, and it can compress the used space.