A soft circular pad, usually made of felt, and placed on a cymbal stand. Felts are used in two locations: underneath the cymbal, they cushion the cymbal and isolate it from its stand. In this location, they may help prevent the cymbal from cracking.
A second felt is sometimes used on top of the cymbal, below the wing nut that secures the cymbal to the stand. In this case, the felt can be pressed down on the top of the cymbal to help control how much it swings and vibrates after being struck.