Definition: Sponge is a single layer of cellular rubber that can be glued between the table tennis blade and the topsheet. If sponge is used, it must be covered with a topsheet. The combination of sponge and topsheet is called a sandwich rubber.
The layer of sponge increases the spin and speed that can be put on the ball. Modern players also apply speed glue or booster to the sponge layer to further increase the spin and speed that can be achieved.