1. variable nounR is the eighteenth letter of the English alphabet.
2. See also three Rs
3. title noun [noun NOUN]R is a written abbreviation meaning king or queen. R is short for the Latin words 'rex' and 'regina'.
...Elizabeth R.
4. R is used as an abbreviation for words beginning with r. For example, it is used on maps as a written abbreviation for 'river'.
5. In the United States, some cinema films are marked R to show that children under 17 years old are only allowed to see them if an adult is with them.