1. (noun) roentgen, R
a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air
2. (noun) gas constant, universal gas constant, R
(physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
3. (noun) R, r
the 18th letter of the Roman alphabet
4. (noun) radius, r
the length of a line segment between the center and circumference of a circle or sphere
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Definition of 'r' 
