A brand name for a type of hardboard. The process was formulated in 1924 and patented in 1926 by American inventor William H. Mason (1877–1940). Painters began using Masonite in the late 1920s as an affordable alternative to wood panel or canvas. Web resources here and here.

Thomas Hart Benton. Spring on the Missouri. Oil and tempera on Masonite. 1945. North Carolina Museum of Art, Raleigh.