Hardboard

Hardboard is an engineered sheet material made by pressing wood fibers, broken down much further than in plywood or particleboard, under heat and pressure into a dense, hard, perfectly uniform panel with no visible grain or layering at all. It's typically thin (1/8" and 1/4" are the common stock thicknesses) and smooth on at least one face, often both.

Its main woodworking uses lean on that combination of thinness, stiffness, and cheapness: cabinet and drawer backs, drawer bottoms, and the perforated version -- pegboard, with regularly spaced holes for hook-mounted tool storage -- are the classic applications. It's not a structural or fine-furniture material; it's a utility panel for backing, bottoms, and templates where flatness and low cost matter more than appearance or strength.