Disk image

A disk image is a computer file containing the complete and exact contents of a storage medium, such as a floppy disk, CD-ROM, DVD-ROM, etc. These can be used for the purposes of backup, bootable drive cloning, or distribution over a network.

Usage
Disk images are often used to install downloadable software and operating system updates onto computers, a task formerly done with physical disks. The .iso extension is often used to identify disk image formats based on the standard defined by the International Organization for Standardization (ISO). Writable disk images can be mounted and accessed like a physical disk on supported operating systems. These can be managed in Mac OS X and macOS through Disk Utility, which also utilizes Apple's proprietary Disk Image (DMG) format.