The distance between the last surface of a lens to its back focal plane.
As measured on the principal axis, from the second lens vertex to the back focal point of the lens. It is not the equivalent of the focal length.
The distance from the final surface of an optical system to the focal plane.
The distance from the surface of the rear-most lens element to the image plane, CCD Sensor Area (video camera) or Film Emulsion (film camera).
The distance from the vertex of the back surface of a lens to the image of an infinitely distant object.