A 'Circle of Confusion' is defined as a small disk that represents the out-of-focus image of a point source in a camera.
This is the definition I was taught 60+ years ago. A 'point' is defined as a 'circle of zero diameteer', as tge image gets further out of focus the diameter of the circle grows, the CoC is the largest circle the eye will see as a point, this sets the limits for DoF, and de0ends on among other things, the magnification needed to produce the print, which is defined in the DoF calculation. This is why viewing at high magnification is an invalid judge of DoF scales.
