R

noun

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
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
3.
the 18th letter of the Roman alphabet
arfulness r