R (http://definr.com/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, R]
     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}, R]
     3: the 18th letter of the Roman alphabet [syn: R]
     4: the length of a line segment between the center and
        circumference of a circle or sphere [syn: radius]