backerboard

noun4

1. •
   A board used behind or under other material, often as support.
2. •
   Boards used as underlayment for tile, plaster, or other materials.

   • A tile setter mounts colorful American-Olean ceramic tile on a new gypsum backerboard in a motel bathroom... — 1962
3. •
   A sacrificial board placed behind or under a board being drilled or sawn, so as to prevent blowouts. (Usually scrap ends reused for such purpose.)
4. •
   Sheets of cardboard or similar material used to preserve or display books, magazines, art prints, etc.