According to Longman Dictionary of Contemporary English Online (http://www.ldoceonline.com/), "overlap" is defined as follows: "if two or more things overlap, part of one thing covers part of another thing"
Can we say that the two plates completely overlap. I wonder whether using "overlap" in combination with words, such as completely and exactly, that have meanings contradicting the meaning (i.e., part) of "overlap" is correct.
Best Answer
"Overlap" by itself is non-specific. The objects can be partially overlapping, or completely overlapping. If it's important you would have to ask the person to clarify which they mean.
I don't think the dictionary is wrong, but it is a little misleading. To be clear the definition should be "part or all of one object covers another object", but technically "part of" something could mean "all of" it. Technically.
If you want to use it as a negative you can say that two objects "do not overlap".