backerboard
Noun
Meaning
A board used behind or under other material, often as support.
(construction, masonry, carpentry) Boards used as underlayment for tile, plaster, or other materials.
Sheets of cardboard or similar material used to preserve or display books, magazines, art prints, etc.
Source: en.wiktionary.orgExamples
Use a trowel to apply thick rows of mortar to the backerboard, then lay the tile sets spacing properly with tile spacers.
For this project, the fireplace opening measured 3 feet by 3 feet, so the backerboard was cut to measure 3 feet and 3 inches by 3 feet and 3 inches.