from
The Free On-line Dictionary of Computing (8 July 2008)
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}]