Back focus is the distance between the back surface of the lens and the image plane , when the lens is focused at infinity.
Background is the area shown behind the main subject in a picture.
Background density is the density of any selection of a negative or print on which there is no image. Also referred to as Fog level . Backing is a dark coating, normally on the back of a film, but sometimes between emulsion and base, to reduce halation . The backing dye disappears during processing. Backlighting is light coming from behind the subject.