“Back focus,” in photography, originally meant what happens when a camera’s focus point is behind the film plane, resulting in soft, out-of-focus images. This usually results from a misaligned lens element or ground glass; the image appears to be in focus when looking through the camera, but on the film, the image is blurry.
The term has morphed in recent years to mean an autofocus error, wherein the camera’s autofocus system latches onto something besides the subject, or just wanders. Modern autofocus systems rely on contrast to work, so back focus most commonly occurs when the photographer is not paying attention to his backgrounds and the matrix gets confused.
Both definitions are valid.
In this section we will permit ourselves to wander into history, technology, and other areas of esoteric and forgotten information.