Fiber protrusion or undercut:

The distance between the fitted domed surface of the ferrule and the polished fiber end is called fiber undercut or fiber protrusion.  If the fiber end is cut beneath the surface of the ferrule, it is said to be undercut.  If the fiber end extends above the ferrule surface, it is said to protrude.  Proper undercut or protrusion allows the fibers to maintain physical contact, while avoiding damage to the fiber itself.  For a UPC connector, the protrusion ranges from +50 to ­125 nm, depending on the radius of curvature.  For the APC connector, the range is from +100 to ­100 nm.

See also: polishing, fiber, interferometer, ferrule, UPC, APC