Circle of Confusion
From Nikonians Wiki - FAQs, Photo Glossary, Good Photo Locations, Help
The circle of confusion has nothing to do with other camera brand users. It is the largest on-film/sensor circle that you can see as a well defined point on an 8×10 print when viewed at from a "normal" viewing distance of 2 to 3 feet. Anything larger is seen as a small circle, not a point and is therefore perceived as out of focus. For 35mm film format the diameter of such point or circle is 0.02501mm, commonly rounded to 0.03mm for hyperfocal distance computations. For APS-C (DX) digital cameras sensors, the CoC is 0.020
