On Unix-like operating systems, like Linux or BSD, a loop device is a regular file or device that is mounted as a file system. It may be thought of as a "pseudo device" because the operating system kernel treats the file's contents as a block device. For example, an ISO file containing internal structure details of files and directories may be mounted as a loop device, and accessed by the operating system, like a physical disk partition.
mount -o image.iso /mnt/myimage
For more information, see mounting loop devices in Linux.