Noun
- 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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