The Jargon File (version 4.4.7, 29 Dec 2003):

vdiff
 /vee'dif/, v.,n.

    Visual diff. The operation of finding differences between two files by 
    eyeball search. The term optical diff has also been reported, and is
    sometimes more specifically used for the act of superimposing two nearly
    identical printouts on one another and holding them up to a light to spot
    differences. Though this method is poor for detecting omissions in the
    ?rear? file, it can also be used with printouts of graphics, a claim few if
    any diff programs can make. See diff.

    An interesting variant of the vdiff technique usable by anyone who has
    sufficient control over the parallax of their eyeballs (e.g. those who can
    easily view random-dot stereograms), is to hold up two paper printouts and
    go cross-eyed to superimpose them. This invokes deep, fast, built-in image
    comparison wetware (the same machinery responsible for depth perception)
    and differences stand out almost immediately. This technique is good for
    finding edits in graphical images, or for comparing an image with a
    compressed version to spot artifacts.


The Free On-line Dictionary of Computing (30 December 2018):

vdiff
optical diff

   /vee'dif/ Visual diff.  The operation of finding differences
   between two files by eyeball search.  The term "optical
   diff" has also been reported, and is sometimes more
   specifically used for the act of superimposing two nearly
   identical printouts on one another and holding them up to a
   light to spot differences.  Though this method is poor for
   detecting omissions in the "rear" file, it can also be used
   with printouts of graphics, a claim few if any diff programs
   can make.  See diff.

   [Jargon File]