| R | the length of a line segment between the center and circumference of a circle or sphere |
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| R | the 18th letter of the Roman alphabet |
| R | (physics) the universal constant in the gas equation: pressure times volume = R times temperature |
| R | a unit of radiation exposure |
| R | a combination of blues and jazz that was developed in the United States by Black musicians |
| R | United States architect who invented the geodesic dome (1895-1983) |
| R | English aeronautical engineer (1895-1937) |
| R | of or relating to or supporting Romanism |
| R | ancient hawk-headed Egyptian sun god |
| R | an intensely radioactive metallic element that occurs in minute amounts in uranium ores |
| R | (astronomy) the angular distance eastward along the celestial equator from the vernal equinox to the intersection of the hour circle that passes through the body |
| R | the capital of Morocco |