On a guillotine cutter, a long, heavy steel blade which is used—either manually or electronically—to cut large stacks of paper or other substrate. See Guillotine Cutter.
On a folding machine—specifically a knife folder, a knife is a metal blade used to force sheets of paper through a set of folding rollers. See Folding and Knife Folder.
The term knife is also used to refer to any of a large number of handheld implements used in graphic arts for cutting friskets (a frisket knife), screen printing stencils (an art knife), or other such work. One popular form of handheld cutting implement is called an X-Acto Knife.