R \R\ ([aum]r).
   R, the eighteenth letter of the 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.
   [1913 Webster]

   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.
         [1913 Webster] The English letter derives its form from
         the Greek through the Latin, the Greek letter being
         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.
         [1913 Webster]

   The three Rs, a jocose expression for reading, (w)riting,
      and (a)rithmetic, -- the fundamentals of an education.
      [1913 Webster]

	

	R
     n 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 [syn: roentgen]
     2: (physics) the universal constant in the gas equation:
        pressure times volume = R times temperature; equal to
        8.3143 joules per kelvin per mole [syn: gas constant, universal
        gas constant]
     3: the 18th letter of the Roman alphabet
     4: the length of a line segment between the center and
        circumference of a circle or sphere [syn: radius]