Circle of Confusion
When a cone of light from a lens does not come to a perfect point on the camera's sensor, it is said to be out of focus, and the resulting circle is called the Circle of Confusion. In practical terms, a very small circle, rather than a precise point can be acceptable, as long as the circle is small enough that the photograph appears to be in focus, and this is called the maximum permissible circle of confusion, the circle of confusion criterion, or the circle of confusion diameter limit, but is quite often incorrectly called simply the circle of confusion.