see = c-mount
Images of point size Objects would ideally be points. For physical reasons (“diffraction”), we only obtain little disks of light.
The so called circle of confusion (also called “Airy disk”) is the smallest disk of light, that an ideal lens can theoretically generate on image side.
The smallest possible diameter of a circular disk that a “perfect” (=diffration limited) lens can generate on image side is given by the Rayleigh Criterion.