WordNet (r) 3.0 (2006):

mouse mat
    n 1: a small portable pad that provides traction for the ball of
         a computer mouse [syn: mousepad, mouse mat]

The Free On-line Dictionary of Computing (30 December 2018):

mouse mat
mouse pad

    (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.

   (1997-04-14)