mat

n. A sheet of cardboard or other materials used in mounting flat items for storage or framing. A simple frame made from cardboard or other material.

Notes

A mat1 commonly has a solid back, on which the item is mounted, and a front with a window cut to allow the item to be viewed. - Mat2 refers to the sheet placed in front of the item; it is distinguished from the mount, which is the backing board to which the item is attached.