A diwan is furniture that is a lot like a couch or a sofa that is laid out on the floor or slightly raised about the ground. A diwan is accessorised with small cushions and bolsters as opposed to how a sofa or a couch would be accessorised.
The diwan got popular from the middle eastern times when a long mattress was laid out along the length of a room against the wall. The cushions and bolsters were meant to rest against while sitting. The term gained popularity from there and got popular in Europe around the mid 18th century.