1. roentgen, R, radioactivity unit

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

2. gas constant, universal gas constant, R, constant

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

3. R, r, letter, letter of the alphabet, alphabetic character

usage: the 18th letter of the Roman alphabet

4. radius, r, length

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

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See also: r (Dictionary)