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1. The 18th letter of the Roman alphabet
   - R
    
2. The length of a line segment between the centre and circumference of a circle or sphere
   - radius

1. A unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air
   - roentgen, röntgen
    
2. (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
   - gas constant, universal gas constant

Derived forms: rs, Rs

Type of: alphabetic character, constant, length, letter, letter of the alphabet, radioactivity unit

Part of: diam, diameter, Latin alphabet, Roman alphabet

Encyclopedia: R, S, T, L, N, E