Circle of confusion
From The Glossary of Digital Photography
When a lens is operating properly, subjects that are perfectly in focus should come to a point at the plane of the sensor. This is known as the focal point of the lens. Subjects closer or farther away will not be in as good of focus as the main subject. Instead of coming to a point at the sensor, the subjects create a small circle or other shape depending on the lens and aperture configuration. If the circle is sufficiently small, the subject is considered to be in focus. As the circle grows larger, due to subject distance from the lens, it becomes more noticeable. This circle is known as a circle of confusion. The range of distances of the subject from the lens when the circle is small enough not to be noticeable is known as the depth of field. The largest circle of confusion still considered acceptably small, such that the subject is in focus, is known as the maximum permissible circle of confusion.
