Definition: a flexible roll-top door or lid that's made of narrow wood slats, glued either horizontally or vertically in parallel formation onto a heavy piece of fabric, traditionally canvas. The edges of the fabric are set into grooves within a piece of furniture. When raised or moved, the fabric rolls around a hidden cylinder, causing it to "retreat" into the top or sides of the piece. Tambours were invented in France - the word is French for "drum" - around the 1760s. They're most commonly seen in roll-top desks, but also appear in other sorts of case furniture, such as cabinets or wash stands, as well as night tables and even laptop writing desks.
Example: Good-quality roll-top desks have two handles on either side of the tambour, to help you pull down the often-heavy lid with ease.