From Longman Dictionary of Contemporary Englishbeer matˈbeer mat noun [countable] DL British English a small piece of cardboard that you put under a glass, especially in a bar
Examples from the Corpus
beer mat• She sat crumbling a beer mat between her fingers.• Penguin has produced a beer mat to help promote it.• 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. 