(U.S.: "mouse pad") A small sheet with a special surface for a rolling ball mouse to move on. Most mouse mats are sheets of rubber or foam about 20cm by 25cm and about 5mm thick with one side covered with cloth or sometimes hard plastic. Deluxe versions come combined with a wrist rest.
It is rare to find a mouse mat which does not carry some form of advertisement for some company or other. They are such a common free gift that few people actually have to buy one.
Mats are supposed to provide better traction and a clean, lint-free surface over which to move but it debatable whether they are useful at all, or whether any appropriate surface (preferably hard, even, flat, and clean) is as good. Howevever, some mice which use optical (e.g. Sun) or radio-frequency sensors (e.g. ?) to detect motion (instead of using a rolling ball) will only work on specially designed mouse mats. Critics may consider this to be part of the connector conspiracy, though the designers would claim greater reliability due to the absence of moving parts.