From Longman Dictionary of Contemporary EnglishRR1, r /ɑː $ ɑːr/ noun (plural R’s, r’s) 1 SLA[countable, uncountable] the 18th letter of the English alphabet2 [singular, uncountable] American English used to describe a film that has been officially approved as only suitable for people over 17 → three Rs
Examples from the Corpus
R• Senator Charles Grassley, R-IowaRR2 1 American EnglishPPG the written abbreviation of Republican, used after a politician’s name to show that he or she belongs to the Republican Party in the US Steve Gunderson (R)2 PG British English the written abbreviation of regina (=queen), used after the name of the queen of the United Kingdom Elizabeth R3 SG (also R.) the written abbreviation of river, used especially on maps → R & B, R & D, R & R 