Plywood is a strong, flat board made by sticking several very thin sheets of wood on top of each other. Each thin sheet is a ply. The sheets are usually laid so the grain — the lines in the wood — runs different ways in different layers. That crossing is the point: plywood does not split as easily as one thick piece of wood, and it stays flatter. Builders use it for floors, walls, roofs, furniture, packing cases, and cheap shelves. You buy it in big boards at a timber yard or hardware shop. It is an everyday materials word, not a fancy one; you hear it on building sites and in DIY talk. People sometimes say plywood for any cheap board, but that is loose. Real plywood is layers of real wood glued together, not the other man-made boards made from chips or dust.