AKA the drawing table, drafting table, architect's table, draughting table, or mechanical desk.
A large rectangular board, mounted on some sort of frame, on which paper is mounted for drawing. They usually look like tall tables, with the top mounted on hinges and levers so that it can raised and lowered, and tilted to an inclined angle.
They are usually used by artists and draftsmen, who want absolute control over their drawing surfaces.
Drawing boards are best known to most of us from expressions like 'back to the drawing board' ("we'll have to start again from the beginning"), and 'still on the drawing board' ("still in the planning stages"), both references to the draftsman drawing up blueprints.
My big old Merriam-Webster's dictionary says that a drawing board is any board that has at least one straight edge against which a T square may be placed, and that is used for drawing on; a very loose definition. It also adds that there is a type of cardboard, AKA bristol board, with a smooth surface suitable for drawing on, that is sometimes called drawing board.