I don't know how you say it, but it's a scanner that is rectangular, and you lift up the lid, put the document face down on the glass, close the lid, and press a button to scan. If you search it in google images, you can see many pictures.
Most of the flatbed scanners talked about in this thread so far have been desktop type devices used for copying.
However, one of the most widely used flatbed scanners are used in POS (Point of Sale) applications in supermarkets and other stores to scan the barcode of the items that the customer is purchasing. These flatbed scanners have no cover and are embedded into the counter. The name flatbed is to distinguish them from vertical scanners and hand-held scanners used in smaller stores.