What is a composition board?

Question: What is a composition board!?

I am doing an essay on 'Shearing the Rams" by Tom Roberts and it says he used oil on canvas on composition board!. i am having trouble finding what a composition board is and it is very important for my essay!.
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Best Answer - Chosen by Asker:

It is a wooden board made from processed wood fibers or different types of wood!. Masonite also known as hardboard is the oldest version !.This gives the canvas support and stability!.Www@QuestionHome@Com