r r /ˈɑːr/

noun

1. The length of a line segment between the center and circumference of a circle or sphere  ( r , radius )
   the length of a line segment between the center and circumference of a circle or sphere
    The radius of the circle was measured to be 5cm
   The radius of the circle was measured to be 5cm
   

 R

1. The 18th letter of the Roman alphabet  ( R )
   the 18th letter of the Roman alphabet
   
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 , R , universal gas constant )
   (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
   
3. A unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air  ( R , roentgen , röntgen )
   a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air