What is the X in X-mas?
/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.