R
       n 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 [syn: roentgen]
       2: (physics) the universal constant in the gas equation:
          pressure times volume = R times temperature; equal to
          8.3143 joules per kelvin per mole [syn: gas constant, universal
          gas constant]
       3: the 18th letter of the Roman alphabet
       4: the length of a line segment between the center and
          circumference of a circle or sphere [syn: radius]