The circle of confusion (CoC) is the blur spot a single point in the scene makes on the sensor or film when it is not perfectly in focus. If that blur spot is small enough to still look like a point in the final image, it is considered “acceptably sharp.”

For depth-of-field calculations, the CoC value used is usually the maximum blur spot size still judged acceptable. That is not a fixed law of optics; it depends mainly on how large the image will be displayed or printed and how closely it will be viewed. Many DoF calculators and tables use standard conventional values rather than something you measure yourself each time.

Important distinction: the actual blur spot created by defocus depends on focus distance, subject distance, aperture, and lens behavior. The acceptable CoC used in DoF tables is a viewing/output standard.

In practice, CoC is mostly used for depth-of-field and related concepts such as hyperfocal distance and acceptable sharpness. It is less about what the sensor “really records” and more about what looks sharp enough to a viewer under assumed viewing conditions.