Circle of Confusion – an out of focus object that is rendered as a blurry disc in a photograph.
From The Camera by Ansel Adams ©1980 volume one of The New Ansel Adams Photography Series pages 49-50:
The image of a “point” in the subject should be a “point” on the film. If the subject is not exactly in the plane of critical focus, however, this image becomes a small blurred disc, called a circle of confusion.* The size of any circle of confusion becomes smaller as the aperture is reduced, making the image appear sharper.
*Even with the finest optics there is no such thing as a “point” image. The image of a true point is always a minute circle because of inevitable diffraction effects of the lens. However, a lens of fine definition gives a minuscule circle or disc which we accept as a “point.”
In practical terms, every image is made up of circles of confusion with some being larger than others. Those that are very small appear in focus and those that are relatively large show up as out of focus discs.