r

(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. Synonyms: roentgen, R.

(noun) (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole. Synonyms: gas constant, universal gas constant, R.

(noun) the 18th letter of the Roman alphabet. Synonyms: R.

(noun) the length of a line segment between the center and circumference of a circle or sphere. Synonyms: radius.