A loop device allows any file to be mounted as a filesystem image. Not to be confused with a loopback interface in networking. Don't use this tag for questions regarding "for-loops" (use the "for" tag instead).
A loop device is a pseudo-device that makes any file accessible through a block device. This way, a file containing a filesystem image can be mounted.
Not to be confused with loopback, which is a network interface, or for-loop, which is a looping construct for performing iterations in code.