From The Collaborative International Dictionary of English v.0.48:

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]

From WordNet (r) 2.0:

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]