Source : Free On-Line Dictionary of Computing

vdiff
     
        /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}]