vdiff

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}.