Definitions for "Backing Board"
A second bottom board on which molds are opened.
General term for the material used to fill the back of the frame; most often scrap matboard or foam-core board. The backing board is held in place by glazier's points or brads and is covered with a dust cover (kraft paper, usually). The English refer to mounting board as "backing board." So, too, do some U.S. conservators.
a rigid board that goes behind the matted item and holds it firmly into the frame
A protective sheet of material attached to the verso of a frame, or sometimes directly to the stretcher, to isolate the back of the support from the environment.