The flange focal distance (FFD) is the distance (along the optical axis) between the flange of the lens mount and the film or sensor (the focal plane), as illustrated by the picture below.
The FFD is a key characteristic of all interchangeable lens cameras, as all of a system’s lenses have to be designed to be able to focus on the film/sensor. Thus (even if you could otherwise mount a lens from a different system onto a camera), the difference in flange focal distance may result in the combination being able to focus only on a part of the lens’ intended focus range.
Example: When mounting an M42 (FFD 45,46 mm) lens on a Nikon F-body (FFD 46,5 mm) using a dumb adapter (ca. 1 mm), the resulting combination is not able to focus on infinity (and then some).
