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.