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

R
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2. \R\ ([aum]r).
   R, the eighteenth letter of English alphabet, is a vocal
   consonant. It is sometimes called a semivowel, and a liquid.
   See Guide to Pronunciation, [sect][sect] 178, 179, and
   250-254. ``R is the dog's letter and hurreth in the sound.''
   --B. Jonson.
   Note: In words derived from the Greek language the letter h
         is generally written after r to represent the aspirated
         sound of the Greek "r, but does not affect the
         pronunciation of the English word, as rhapsody,
         rhetoric. The English letter derives its form from the
         Greek through the Latin, the Greek letter derived from
         the Ph[oe]nician, which, it is believed, is ultimately
         of Egyptian origin. Etymologically, R is most closely
         related to l, s, and n; as in bandore, mandole; purple,
         L. purpura; E. chapter, F. chapitre, L. capitulum; E.
         was, were; hare, G. hase; E. order, F. ordre, L. ordo,
         ordinis; E. coffer, coffin.
   {The three Rs}, a jocose expression for reading, (w)riting,
      and (a)rithmetic, -- the fundamentals of an education.