r
1 (n)
the length of a line segment between the center and circumference of a circle or sphere
2 (n)
the 18th letter of the Roman alphabet
3 (n)
(physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
4 (n)
a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air