Longman Dictionary of Contemporary English
beer mat
noun
EXAMPLES FROM CORPUS
▪ It was more like a saucer or a small beer mat.
▪ My eyes glued to the beer mat I tried to take in what was being said.
▪ Penguin has produced a beer mat to help promote it.
▪ She sat crumbling a beer mat between her fingers.
Wiktionary
beer mat
alt. 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. n. 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.
WordNet
beer mat
n. a drip mat placed under a glass of beer