r

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The 18th letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter r.
• noun The 18th in a series.
• noun Something shaped like the letter R.
• abbreviation radius
• abbreviation Electricity resistance

from The Century Dictionary.

• noun An abbreviation of Royal Academy;
• noun Royal Academician;
• noun Royal Arch;
• noun right ascension.
• noun An abbreviation of Royal Dragoons;
• noun of Rural Dean.
• noun An abbreviation of Roman Catholic.
• noun An abbreviation of Royal Engineers;
• noun of Royal Exchange.
• An abbreviation of Royal Marines;
• Royal Mail;
• Resident Magistrate.
• An abbreviation of Royal Navy.
• noun An abbreviation of Right Worshipful; Right Worthy.
• noun An abbreviation of Revised Version (of the Bible).
• The eighteenth letter and fourteenth consonant in the English alphabet, representing a character having a like position and value in the alphabets from which the English is derived—the Latin, Greek, and Phenician.
• The tag below the curve by which the English (and the Latin) R differs from the later (Greek form P was added to the latter in order to distinguish it from the p-sign after this had assumed its present form; the addition was first made on Greek ground, but was abandoned there when the distinction of the p- and r-signs had become established in another way. The value of the character has always been essentially the same; it represents a continuous sonant utterance made between the tip of the tongue and the roof of the mouth, at a point more or less removed backward from the upper front teeth. The sound is so resonant and continuable as to be nearly akin with the vowels; and it is, in fact, used as a vowel in certain languages, as Sanskrit and some of the Slavic dialects: in normal English pronunciation, however, it never has that value. By its mode of production it is nearly akin with l, and r and l are to a large extent interchangeable with one another in linguistic history. It is often classed as a “liquid,” along with l, m, n; less often, but more accurately, as a semivowel, with l, y, w. It also, on no small scale, answers as corresponding sonant (in languages that have no z) to s as surd, and comes from s under sonantizing influences: so in Sanskrit, in Latin (as ara from asa), and in Germanic (as in our were, plural of was). In Anglo-Saxon the initial r of many words was aspirated (that is, pronounced with an h before it), as hring (our ring); but the aspiration was long ago abandoned, both in pronunciation and in spelling. In Greek initial r was always thus aspirated, and the combination was transliterated in Latin by rh instead of hr: hence the frequency of rh iu our words of Greek derivation. Moreover, such an r, when by inflection or composition made medial, became rrh, and double r was in general viewed as rrh: whence that spelling in many of our words (for example, diarrhea, hemorrhage, catarrh, etc.): in recent scientific words and names taken from Greek, the Greek rule and Latin practice as regards the doubling and aspiration of the r are often neglected. The mode of production of the r-sound itself varies greatly in different languages and dialects. Normally its utterance is combined with a distinct trilling or vibration of the tip of the tongue, in various degrees (the sound is thence often called the “dog's letter,” littera canina). But in ordinary English pronunciation this vibration is either extremely slight, or, more commonly, altogether wanting; in fact, the tip of the tongue is drawn too far back into the dome of the palate to admit of vibration; the English r is a smooth r. But further, in many localities, even among the most cultivated speakers, no r is ever really pronounced at all unless followed (in the same word, or, if final, in the word following) by a vowel (for example, in are, farther, pronounced ah, father); it either simply disappears, or, as after most long vowels, is replaced by a bit of neutral-vowel sound, of ũ or é; and after such a long vowel, if it comes to be pronounced by the addition of a vowel, it retains the same neutral-vowel sound as transition-sound (for example, in faring, fearing, pouring, during, firing, souring: the pronunciation is indicated in this work by retaining the r in the same syllable with the long vowel: thus, fār′ ing, fēr′ tng, etc.). An r has a stronger and more frequent influence upon the character of the preceding vowel than any other consonant; hence the reduction to similarity of the vowel-sounds in such words as pert, dirt, curt, earn, myrrh. If all our r's that are written are pronounced, the sound is more common than any other in English utterance (over seven per cent.); the instances of occurrence before a vowel, and so of universal pronunciation, are only half as frequent. There are localities where the normal vibration of the tip of the tongue is replaced by one of the uvula, making a guttural trill, which is still more entitled to the name of “dog's letter” than is the ordinary r: such are considerable parts of France and Germany; the sound appears to occur only sporadically in English pronunciation.
• As a medieval Roman numeral, 80, and with a line over it (), 80,000.
• As an abbreviation:
• Of Rex or Regina, as in George R., Victoria R.
• Of Royal, as in R. N. for Royal Navy, R. A. for Royal Academy or Academician, or for Royal Arch (in freemasonry).
• Prefixed to a medical prescription (℞), of recipe, take.
• Nautical:
• In a ship's log-book, of rain.
• When placed against a man's name in the paymaster's book, of run away.
• Of right (right-hand), as in R. A. for right ascension, R. H. E. for right second entrance (on the stage of a theater).
• In math., r is generally a radius vector of coordinates, R the radius of a circle, ρ a radius of curvature.
• noun An abbreviation of Rear-Admiral;
• noun of Royal Arcanum;
• noun of Royal Artillery (also R. Art.).
• noun An abbreviation of Red Cross
• noun [lowercase] of right center.
• noun In electrotherapy, an abbreviation of reaction of degeneration.
• noun An abbreviation of Rifle Brigade.
• noun An abbreviation of Reformed Episcopal
• noun of Right Excellent
• noun [lowercase] of right eye.
• noun An abbreviation of Rhode Island.
• noun An abbreviation of right hand;
• noun [capitalized] of Royal Highness.
• noun An abbreviation of rapid fire;
• noun in mineralogy, of reducing flame;
• noun [capitalized] of République Française, French Republic;
• noun [capitalized] of Rex Francorum, King of the Franks (French).