why is it called "loopback" device?

from http://stackoverflow.com/questions/3614719/what-does-loop-mean-in-loop-device

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Why is it called "loopback" though? I can see two parallels: Loopback of the network device (i.e having some word play), or loop back in the sense that the driver asks back into the file-system again for the file to map into. What meaning is correct? – Johannes Schaub - litb Sep 1 '10 at 4:02

I think it's the latter one. Correct me if I am wrong. – smwikipedia Sep 1 '10 at 4:14

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It's the latter one. Normally a request would go userspace -> VFS -> block device layer, but when the block device driver is loop, it goes userspace -> VFS -> loop device -> VFS -> block device. So it loops back to a higher level. – hobbs Sep 1 '10 at 4:19

As far as I know the term originated in the depths of the telecom industry (and probably dates back to telegraphs). See mmonem's answer. – RBerteig Sep 1 '10 at 6:44

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I guess this term comes from the communication realm when sometimes it is needed to test the communication system by simulating a peer using a proxy circuit loop.
The concept came also to UNIX networking where loopback network interfaces do not send network traffic to the medium.
The same concept in file systems loop means that the file system driver does not really goes through the hard disk IO stack and, instead, ends using a plain disk image file for IO.

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