Definitions of R:

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