… blur spot … size on image sensor …
Instead of moving a subject back and forth and seeing how the size of the blur spot is affected, we can look at it the other way, define a maximum size of the spot that we are willing to accept as sharp, and see how far we can move our subject such that the spot never exceeds this size. This maximum diameter is called the circle of confusion c.
There is much debate on how the circle of confusion should be determined, which we will ignore for the moment. Just think of it as a small value such as 0.033 mm (one thirtieth of a mm, or 0.0013 inches).
Merklinger (2002)