r \r\ aumr.
   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, sectsect 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 phoenician, 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, writing,
      and arithmetic, -- the fundamentals of an education.
      1913 webster
see also:
the three rs 

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
see also:
roentgen gas constant universal gas constant radius