Any flat (as opposed to cylindrical) surface used to mount a printing plate or typeform in letterpress printing. See Letterpress: Flatbed Press and Letterpress: Platen Press.
In imaging, the term flatbed is descriptive of any optical input or output device—such as a scanner or imagesetter—that images the original from a flat surface (rather than a cylinder) or exposes film or paper on a flat surface. See Flatbed Imagesetter, Flatbed Plotter, and Flatbed Scanner.