On an offset lithographic printing press cylinder (in particular, the plate cylinder and the blanket cylinder), the difference between the radius of the cylinder's bearers (metal rings at each end of the cylinder) and that of the cylinder body itself. The cylinder body—the portion of the cylinder where the plate or blanket is attached—is lower than that of the "true" diameter of the cylinder (indicated by the diameter of the bearer). The cylinder is undercut so as to provide room for plate or blanket packing, or adjustments in the height of the plate or blanket. (See Plate Cylinder, Blanket Cylinder, and Packing.) On the press impression cylinder, it is typically the bearers which are undercut, as the impression cylinder does not require packing.
In cutting and trimming, an undercut is a short undesirable cut caused by weak pressure of the cutter clamp. See Cutting and Trimming.