Definition of "hardboard"

The compartment ceiling panels are of plastic material backed with ply or hardboard panels.

1951 May, “British Railway Standard Coaches”, in Railway Magazine, page 328

For convenience and versatility, no mass-produced wood paneling can beat hardboard—a tough, pliable, water-resistant material made from wood fibers.

1980, Robert M. Jones, editor, Walls and Ceilings, Time-Life Books, page 67