beer mat

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small square cardboard mat, often with an advertisement for a brewery on it, that is used to rest one's glass on, and to protect the surface of a table or a bar in a pub.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a drip mat placed under a glass of beer