WordNet sense 1 (a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air):

roentgen

WordNet sense 2 ((physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole):

gas constant, universal gas constant

WordNet sense 3 (the 18th letter of the Roman alphabet):

letter r

WordNet sense 4 (the length of a line segment between the center and circumference of a circle or sphere):

radius