Etymology
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retrovirus (n.)

1977, earlier retravirus (1974), from re(verse) tra(nscriptase) + connective -o- + virus. So called because it contains reverse transcriptase, an enzyme that uses RNA instead of DNA to encode genetic information, which reverses the usual pattern. Remodeled by influence of retro- "backwards."

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jingle (n.)
"tinkling or clinging sound," such as made by small bells, 1590s, from jingle (v.). Meaning "something that jingles" is from 1610s, especially "metallic disc on a tambourine." Meaning "song in an advertisement" first attested 1930, from earlier sense of "catchy array of words in prose or verse" (1640s).
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rhythm (n.)

 16c. spelling variant or attempted classical correction of Middle English rime "measure, meter, rhythm," also "agreement in end-sounds of words or metrical lines, rhyme; a rhyming poem" (12c.), from Old French rime "verse," from Latin rhythmus "movement in time," from Greek rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement, order; form, shape, wise, manner; soul, disposition," related to rhein "to flow" (from PIE root *sreu- "to flow"). Compare rhyme.

The word rhyme has no connection with the word rhythm, nor is rhyme necessary to accentual verse. Nevertheless, rhyme was usually present. On the other hand, in classical Greek metrical poetry, rhymes, if not accidental, were never an essential element of metrical verse structure. [Henry Osborn Taylor, "The Classical Heritage of the Middle Ages," 1911]

The spelling fluctuated 16c.-17c., rithme and ri'me also being used. From 1550s as "metrical movement, movement in time characterized by equality of measures and alteration of stress and relaxation." By 1776 as "regular succession of beats or accents in music."

The rhythm method in reference to birth control is attested from 1936. Rhythm and blues, U.S. music style, is from 1949 (first in Billboard magazine).

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cinquain (n.)

"collection of five," 1711, from French cinquain "bundle of five objects," from cinq "five" (from PIE root *penkwe- "five"). Originally in English of military orders of battle (five battalions drawn up in three lines); of five-lined stanzas of verse from 1882 (give a more specific form in English than usual in French).

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marchen (n.)

"German fairy or folk tale," 1871, from German Märchen, "a story or tale," from Middle High German merechyn "short verse narrative," from Old High German mari "news, tale," from Proto-Germanic *mērijaz "renowned, famous, illustrious" (source of Old English mære "famous, renowned," Old Saxon mari,  Dutch maar, mare) + diminutive suffix -chen.

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metric (n.)

"science of versification," 1760, from Latinized form of Greek he metrikē "prosody," plural of metron "meter, a verse; that by which anything is measured; measure, length, size, limit, proportion" (from PIE root *me- (2) "to measure"). Middle English had metrik "the branch of music which deals with measure or time" (late 15c.), from Medieval Latin metricus.

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leonine (adj.)
"lion-like," late 14c., from Old French leonin or directly from Latin leoninus "belonging to or resembling a lion," from leo (genitive leonis) "lion." Weekley thinks that Leonine verse (1650s), rhymed in the middle as well as the end of the line, probably is from the name of some medieval poet, perhaps Leo, Canon of St. Victor, Paris, 12c.
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pentameter (adj.)

"consisting of five metrical feet," 1540s, from French pentametre, from Latin pentameter, from Greek pentametros (adj.) "having five measures," from pente "five" (see five) + metron "measure" (from PIE root *me- (2) "to measure"). As a noun from 1580s, "a verse line of five feet;" in ancient prosody "a dactylic dipenthemimeres or combination of two catalectic dactylic tripodies" [Century Dictionary].

The verses we have hitherto examined may be constructed at pleasure of any kind of metre—dactyl, troche, iamb, or anapest. But all at once, we now find this liberty of choice refused. We may write a pentametre verse in iambs only. A most notable phenomenon, significant of much more than I can at present understand,—how much less explain .... [Ruskin, "Elements of English Prosody, for use in St. George's Schools," 1880]
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rhyme (n.)

"agreement in terminal sounds of words or metrical lines," a 16c. attempt to restore a classical spelling to Middle English ryme, rime (c. 1200) "measure, meter, rhythm," later "rhymed verse" (mid-13c.), from Old French rime (fem.), which is related to Old Provençal rim (masc.), earlier *ritme, from Latin rithmus, from Greek rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement, order; form, shape, wise, manner; soul, disposition," related to rhein "to flow" (from PIE root *sreu- "to flow").

The persistence of rime, the older form of the word, perhaps is due to popular association with Old English rim "number" (from PIE root *re- "to reason, count"). The intermediate form rhime was frequent until late 18c.

In Medieval Latin, rithmus was used for accentual, as opposed to quantitative, verse, and accentual verse usually was rhymed, hence the sense shift. In prosody, specifically the quality of agreement in end-sounds such that the last stressed vowel, and any sounds after it, are the same, and preceding sounds differ. 

Verse was invented as an aid to memory. Later it was preserved to increase pleasure by the spectacle of difficulty overcome. That it should still survive in dramatic art is a vestige of barbarism. [Stendhal "de l'Amour," 1822]

The sense of "a piece of poetry in which consonance of end-sounds is observed" is from 1610s. From 1650s as "word that rhymes with another." The phrase rhyme or reason "good sense" (chiefly used in the negative) is from late 15c. (see reason (n.)). Rhyme scheme "ordered pattern of end-rhymes in metrical composition" is attested from 1931. Rhyme royal (1841) is a stanza of seven 10-syllable lines rhymed a-b-a-b-b-c-c.

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rhythmic (adj.)

c. 1600, "pertaining to rhythm in art," from French rhythmique or directly from Latin rhythmicus, from Greek rhythmikos, from rhythmos "measured flow or movement, rhythm; proportion, symmetry" (see rhythm). By 1630s of language, verse, music, etc., "marked by regularly recurring accents or beats," especially if strong or smooth. Related: Rhythmical (1560s); rhythmically.

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