definition of
r

[1] #1 r n. [roentgen, R] a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air. wn

[2] #2 r n. [gas constant, universal gas constant, R] (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole. wn

[3] #3 r n. [R] the 18th letter of the Roman alphabet. wn

[4] #4 r n. [R] the 18th letter of the Roman alphabet. wn

[5] #5 r n. [radius] the length of a line segment between the center and circumference of a circle or sphere. wn