baking stone [beyk-ing stohn] noun
A flat stone (sometimes round or square/rectangle) that can be placed in an oven and pre-heated to mimic the action of a professional stone-lined baker’s oven.
You can bake pizza and bread directly on the stone for crisp and desirable results. Many bakers leave their stone in the oven 24/7. You can find a great version at King Arthur Flour.