Back focal length (BFL) or Back focal distance(BFD) is the distance from the vertex of the last optical surface of the system to the rear focal point (where light beams parallel to the lens optical axis are crossed). This value determines possibility of using the lens in different camera types, and should be measured with 0.01mm accuracy.
The back focal length is shorter than focal length usually. But due to wide spread of reflex cine cameras with inclined mirror shutter, special measures applied in calculation of wide angle lenses to increase their BFL enough to allows their usage on reflex cameras. BFL of such lenses may be much longer than focal length. For example, OKS1-16 lens has focal length of 16mm, but BFL of 24.8mm.