millboard

/ˈmɪlˌbɔrd/
noun
  1. A strong, stiff cardboard or pasteboard used for book covers, boxes, or other sturdy items.
    • The old book had covers made of thick millboard that had lasted for decades.
    • The packaging company ordered a large shipment of millboard for making boxes.
    • She used millboard to create a sturdy backing for her art project.