r

noun

1. 1

   the 18th letter of the Roman alphabet

2. 2

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

3. 3

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

4. 4

   a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air