cardboard; paperboard; pasteboard
Refers to thick, stiff paper used for boxes, covers, and packaging. More specific than 段ボール (corrugated cardboard).
この箱は板紙でできている。
This box is made of cardboard.
板紙は包
Paperboard is widely used as a packaging material.
ivory paper
Compound of 板 (ita, 'board') and 紙 (kami, 'paper'), literally 'board paper'. The reading いたがみ is a straightforward combination.