Princeton's WordNet

1. roentgen, 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:
   gas constant, radius, roentgen, universal gas constant

2. gas constant, universal gas constant, 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, radius, roentgen, universal gas constant

3. R, r(noun)

   the 18th letter of the Roman alphabet

   Synonyms:
   gas constant, radius, roentgen, universal gas constant

4. radius, r(noun)

   the length of a line segment between the center and circumference of a circle or sphere

   Synonyms:
   r, radius, gas constant, universal gas constant, roentgen, spoke, wheel spoke